I'm guessing you're referring to the answer on grenadiers I wrote earlier today. I should perhaps have been more clear in my definitions.

I don't have time to write a comprehensive answer right now, and I'm sure there are other redditors here better placed to elaborate on the use of shock tactics in a macro perspective, but here goes: In the purest sense, shock troops rely on the combined use of physical force and presence in order to defeat an enemy force - usually by causing enemy morale to collapse, engendering a rout or surrender. The term "shock" thus refers both to the direct physical impact of arms, but also - more importantly - to the psychological aspect of "shocking" the enemy.

The term is used to differentiate close physical force from that of longer range firepower. The difference becomes important with the exponential growth of firepower available to armies as a whole, but also to individual soldiers, over the course of the early modern period. Western battlefield tactics during the late 18th and early 19th centuries relied on a combination of both these "weapons" to achieve success. Although both grenadiers and ordinary line infantry would have been capable of dealing both - relying on the musket for firepower and bayonet for shock - grenadiers were trained and equipped to maximise the amount of shock delivered to an enemy force. The disciplined advance, large stature of grenadiers, their tall caps and ferocious appearance would all contribute to this. The same goes for other contemporary units which specialised in the use of shock to achieve results, most notably heavy cavalry or lancers. Achieving the maximum amount of shock was important, because close combat was exceedingly dangerous and costly. A grindy melee would be the rare exception, not the rule. Shock would therefore seek to overwhelm the enemy force as quickly as possible.

Likewise there have been many attempts to find way by which the impact of shock delivered by the enemy could be minimised or negated. The shield wall you mentioned in your question above, as well as discipline, fortifications, firepower and much more have all been used.

It is less common to distinguish between shock and firepower before the early modern period, for the simple reason that firepower was, with a few notable exceptions, much less of a deciding factor on the battlefield. In this context I should perhaps note that distance fire can also be a shock weapon. The republican/early-imperial Roman use of a heavy pila volley from short range just before a charge is a good example of how ranged and melee arms would combine to maximise shock. 18th century line infantry would use similar tactics in delivering a musket volley at close range before following it up with an immediate bayonet charge.

Despite the massive growth and importance of firepower, shock would remain a viable weapon throughout the Napoleonic and into the modern period, as new tactics were developed by which it could be delivered. The column of attack (or l'ordre profond) that I mentioned in the grenadier thread being one of them.