Basically nothing like you'd expect from the most common interpretation. If you were in a dense formation with your buddies and were determined to stand your ground, you likely wouldn't be charged at all (from the front anyway). If you were a bunch of ragtag peasants looking terrified, you'd likely break at the first sign of heavy cavalry and be unceremoniously trampled into the dirt. The outcome relies almost entirely on the discipline of the infantry on the receiving end, their formation, and their armament.

Cavalry, even "shock" cavalry, were not used to plow into a prepared front of infantry. It would be suicidal for the horse and rider. If they went through with the charge, if the horse hit you directly it could very likely be fatal, and the following horsemen would have a good chance of trampling you to death if the initial hit didn't work. These riders would be in incredible danger though, assuming your brothers-in-arms held firm. Each rider would be incredibly vulnerable in melee combat until they could break through. The deeper/denser the infantry formation, the less likely it would work. Additionally, if the horse was wounded there was a high risk of being thrown, which in itself could be fatal.

The purpose of heavy cavalry was to engage other cavalry, threaten flanks, and run roughshod over unprepared/wavering infantry that would break in fear. Often heavy cavalry would feign a charge to see the reaction of the opponent. If they would start to break, they would charge home and send the opponent fleeing. Additionally, if a weak point in the enemy line presented itself, disciplined cavalry would often hammer that point to attempt to break the opponent.

In the case where the enemy's cavalry was absent or removed from the field, they would commonly attack the enemy's camp or the rear of the army and attempt to shatter it. Additionally, they were great for running off reserves/archers.

There are very few instances of super-heavy cavalry charging home against a solid wall of infantry, and it basically had very little effect. The most famous charges in this period almost all involved the enemy shattering in the face of the charge, rather than resisting it. A successful charge was capable of devastating damage to the opposing army.

Some examples would be the Romans fighting against Parthian cataphracts. The dense blocks of Romans forced the Parthians to apply absolutely withering arrow fire for long periods of time before the legions were ready to crack before engaging with the cataphracts. These were infantry armed primarily with short swords for defense, showing that a spear was not necessary to deter cavalry charges. They also liked to chuck spears at charging cavalry to break up the momentum of a charge, but these would usually be spent early in the battle.

Charles Martel at Tours also showed that heavy, well-disciplined infantry could not be broken by cavalry as long as they held to their tight formation and their flanks were protected.

At the battle of Hastings the Saxons repulsed multiple Norman cavalry charges by staying in formation, only failing when they broke ranks to pursue the enemy.

At the Battle of Dyrrhachium, the Varangian Guard successfully stood their ground against a Norman cavalry charge, but were run off by infantry attacking their flank. The cavalry would later be used to rout the rest of the army.

At Falkirk, the scots were able to repel English cavalry charges by holding to their schiltron formations. Similar result at Bannockburn.

At Golden Spurs, the French tried to rout the Flemish with cavalry charges. Although some of them managed to break through, they were summarily slaughtered, being surrounded by heavily armed infantry.

These failed attempts at a shock cavalry "punch-through" began the trend of European knights largely fighting on foot. This came with its own drawbacks, and charges would still be used from time to time, but the illusion of unbeatable cavalry charges was long dead. The rise of professional soldiery also added to this dilemma. The battlefield elite were not just the knights, but the professional infantry of the late middle ages wearing quality armor and thoroughly trained (and paid) to fight against cavalry.

That doesn't mean they stopped using them. It just means they were very carefully calculated. If an army had a very well disciplined and armed/armored cavalry contingent, they could be used to break an enemy very quickly given the right conditions. At the battle of Muret, a group of some 1300 crusaders defeated an army of 40,000 by smashing the opponents' cavalry in a charge and killing the opposing king, causing the entire army to crumble.

Sources:

A History of Warfare – John Keegan

European Medieval Tactics, Vol. 1: The Fall and Rise of Cavalry 450-1260 - David Nicolle

Anglo-Norman Warfare - Boydell

The Face of Battle - John Keegan

War in the Middle Ages - Philippe Contamine

As has already been mentioned, instances of cavalry of any type charging at full pace into a steadfast infantry formation are exceedingly rare. In fact the only one I can think of with a first hand account isn't a medieval battle at all, and was accidental. This charge occurred during the Battle of Omdurman in 1898. What makes this particular battle relevant is that there was a cavalry charge against a dense infantry formation AND we have a first hand account by none other than Winston Churchill who was writing for the Morning Post at the time, here is an excerpt:

Two hundred and fifty yards away the dark-blue men were firing madly in a thin film of light-blue smoke. Their bullets struck the hard gravel into the air, and the troopers, to shield their faces from the stinging dust, bowed their helmets forward, like the Cuirassiers at Waterloo. The pace was fast and the distance short. Yet, before it was half covered, the whole aspect of the affair changed. A deep crease in the ground---a dry watercourse, a khor---appeared where all had seemed smooth, level plain; and from it there sprang, with the suddenness of a pantomime effect and a high-pitched yell, a dense white mass of men nearly as long as our front and about twelve deep. A score of horsemen and a dozen bright flags rose as if by magic from the earth. Eager warriors sprang forward to anticipate the shock. The rest stood firm to meet it. The Lancers acknowledged the apparition only by an increase of pace. Each man wanted sufficient momentum to drive through such a solid line. The flank troops, seeing that they overlapped, curved inwards like the horns of a moon. But the whole event was a matter of seconds. The riflemen, firing bravely to the last, were swept head over heels into the khor, and jumping down with them, at full gallop and in the closest order, the British squadrons struck the fierce brigade with one loud furious shout. The collision was prodigious. Nearly thirty Lancers, men and horses, and at least two hundred Arabs were overthrown. The shock was stunning to both sides, and for perhaps ten wonderful seconds no man heeded his enemy. Terrified horses wedged in the crowd; bruised and shaken men, sprawling in heaps, struggled, dazed and stupid, to their feet, panted, and looked about them. Several fallen Lancers had even time to remount.

Meanwhile the impetus of the cavalry carried them on. As a rider tears through a bullfinch, the officers forced their way through the press; and as an iron rake might be drawn through a heap of shingle, so the regiment followed. They shattered the Dervish array, and, their pace reduced to a walk, scrambled out of the khor on the further side, leaving a score of troopers behind them, and dragging on with the charge more than a thousand Arabs. Then, and not till then, the killing began; and thereafter each man saw the world along his lance, under his guard, or through the back-sight of his pistol; and each had his own strange tale to tell.

Stubborn and unshaken infantry hardly ever meet stubborn and unshaken cavalry. Either the infantry run away and are cut down in flight, or they keep their heads and destroy nearly all the horsemen by their musketry. On this occasion two living walls had actually crashed together. The Dervishes fought manfully. They tried to hamstring the horses. They fired their rifles, pressing the muzzles into the very bodies of their opponents. They cut reins and stirrup-leathers. They flung their throwing-spears with great dexterity. They tried every device of cool, determined men practiced in war and familiar with cavalry; and, besides, they swung sharp, heavy swords which bit deep. The hand-to-hand fighting on the further side of the khor lasted for perhaps one minute. Then the horses got into their stride again, the pace increased, and the Lancers drew out from among their antagonists. Within two minutes of the collision every living man was clear of the Dervish mass. All who had fallen were cut at with swords till they stopped quivering, but no artistic mutilations were attempted. The enemy's behavior gave small ground for complaint.

Morning Post – Thursday 29 September 1898

Churchill mentions that this type of occurrence were rare during his time and this is generally accepted as true for most time periods.

There is one other type of charge that I am familiar with. Not quite the dramatic full speed, almost suicidal effort to break through the enemy line, but a maneuver of heavy cavalry, moving at a trotting pace, pushing their way through the infantry formation. This type of maneuver is described in the Praecepta militaria by Nikephoros II Phokas written in the tenth century AD.

At this point the commander of the prokoursatores must dispatch fifty of this men through the two intervals on either side of the kataphraktoi out to the right flank of the kataphraktoi and fifty out to the left to ride beside the kataphraktoi and keep the enemy away from their flanks so that they do not divert or disrupt the kataphraktoi and break up their charge.

The kataphraktoi and the two units on either side of them must remain in formation while our units make their attack against them, as soon as the enemy's arrows begin to be launched against the front of the kataphraktoi, our archers must strike back at the enemy with their arrows. Then the front of the triangular formation must move in proper formation at a trotting pace and smash into the position of the enemy commander while the prokousatores on the outside encircle the enemy as far as possible and the and the other two units of proceed on both flanks with perfect precision and evenness with the rear ranks of the kataphraktoi without getting too far ahead or breaking rank in any way. With the aid of God and through the intercession of His immaculate Mother the enemy will be overcome and give way to flight.

Note there is no mention of charging the flanks or rear of the enemy with the kataphraktoi or heavy cavalry. The kataphraktoi are to aim their formation where the enemy general is standing and push through the formation at a trotting pace, not a gallop. There is also no mention of feinting charges or pulling back if the infantry stand their ground, the intent was to break the enemy formation and run them off the field, regardless of whether or not the infantry stood and fought or not. I'm not saying these types of feints didn't happen, just that Nikephoros doesn't make any mention of them in his battlefield tactics.

The Praecepta also mentions how the Romans (you can also call them Byzantines but they're Romans dammit) defended against heavy cavalry charges. Specifically how special infantry called Menavlatoi (I've also seen this translated as Menaulatoi), were used.

[the Menavlatoi] must have thick menavlia with a length of one and a half or two ourguia while their points must have a length of one and a half or two spithamai, so that, when battle begins, the units will calmly move out through the intervals and engage the enemy.

The maneuver Nikephoros Phokas is describing is when heavy cavalry are preparing for a charge against the Byzantine formation, the Menavlatoi take position in front of an extra rank of spearmen (for a total of 3 ranks of spears + 1 rank of Menavlatoi) to stand firm against the cavalry charge. Their purpose, along with the description points to some type of pike or halberd, maybe a bill sort of weapon. Regardless of what it was, it was the front line of defense against a heavy cavalry charge.

Nikephoros Phokas continues:

Their menavlia must not be made from wood cut into sections but from saplings of oak, cornel, the so called artzekidia, of from another hard wood. if trees in one piece cannot be found, let these menavla be made from trees cut into sections, but they have to be of hard wood and as thick as hands can wield

This is also interesting, if the Romans didn't expect heavy cavalry charges, why bother with cumbersome Menavlia? Why not just use normal spears like the majority of the infantry? I believe this is because the enemy would charge into well ordered ranks of infantry, likely not at a fast pace but charge nonetheless. The menavlia were essentially small trees with blades on the end, designed to completely stop this type of advance in its tracks and protect the rest of the infantry formation.

edits for spelling and formatting