I'm doing some fictional writing, and I was wondering how mediaeval commanders might order the soldiers around. Any lexis they might have been used would be very useful. For e.g I'm already aware of nock, draw, loose.
In one scene particularly an army is retreating, then reinforcements arrive and begin attacking the enemy from a separate direction. What might the commander say to negate the retreat, get his men back into formation, and advance once more?
For starters, I'm not aware of anyone ever using nock-draw-loose as an actual chain of commands. There were orders for volley fire and fire at will, but not like that, if only because making someone hold a 130lbs warbow drawn is a supremely stupid idea.
Now, your average medieval commander will not have a lot of ways to stop a retreat - you need a supremely well-disciplined army to do that, and even then, it's not a sure thing. We're talking the levels of crusading orders (Templar, Hospitaller, Teuton) or extremely elite mercenaries (Black Army of Matthias Corvinus) who drill all day every day to be able to do this.
What you need to understand, and what you won't easily understand, is the sheer chaos that goes on when two lines of soldiers clash. Every individual soldier has his hands full with not dying, there's a lot of noise, metal clashes against metal, people shout and yell and that's a modern reconstruction - add to that the moans and screams of dying men and animals for a real battle.
Add to that that padding under your helmet and maybe even helmet itself is over your ears, and you can't hear much - you need to raise your voice to even talk properly once the helmets are on. A soldier directly engaging enemy will therefore not hear much in the way of orders. This fact is sometimes alluded to, rule of the Teutonic knights allows field commanders to use their mace to make soldiers notice/obey them in battle (during training, he has to use a 'rod', no, no one really knows what that means).
A medieval commander will therefore mostly give maneuvering orders and then either order to hold fast, advance in formation or charge. If he wants to have retreating/routing men to stay in battle, he has to create a strong point for them to rally around - this was usually the role of banners and their bearers, they told their own men where friendlies were, and where they could regroup.
In your situation, assuming it's a somewhat organized retreat swiftly breaking into a rout (e.g. first wave of your soldiers failed, you see that there is too much enemy forces and order the retreat of the second wave waiting in reserve, to screen the routing men from the first), shouting that "Duke such and such has come! Let's get them!" and letting the good news spread from soldier to soldier could do the trick. Leading by example would also help, since medieval commanders were knights - on horse and brightly decorated.
how mediaeval commanders might order the soldiers around.
In the 13th century invasion of Sweden by Norway, we see communication was handled via several smaller cavalry companies between each goal post, allowing each horse to sprint short distances. ~80 men, at maximum ~6 km, then 30, then finally 8 closest to the king. There is also description of each man having two horses and extra horse handlers, but it's unclear how exactly this functions, because the author didn't describe it thoroughly. Since it's fiction, and if you want to be a bit more creative as to how different subdivisions communicated over distance, then you can always do something similar to this depiction from Germany. A blunt crossbow bolt with a letter attached.
The first instances that come to mind for your particular example was the Battle of Hastings. Here, we observe the term feint attack, feint retreat or feigning flight, as William of Malmesbury wrote.
Observing this, William gave a signal to his troops, that, feigning flight, they should withdraw from the field.
We see the same wording from William of Newburgh when describing King Sverre's usurpation of the Norwegian throne from the former King of Norway. Here he is describing how Sverre, who he dislike, managed to win ground via smaller mobile forces against the larger forces of the old aristocrat.
[3] After having for a time ministered in the sacred order which he had received in the church, this truly daring and crafty man, through extreme confidence in himself, began to aspire to the kingdom. Soon after, he surveyed the whole district, and diligently advanced the business of his meditated project by artfully assembling around him a band of bold and desperate men, hired by the hope of plunder; and availing himself of trackless deserts as fortresses, he annoyed the king by perpetual irruptions; and when the sovereign pursued him with his army in array, he, artfully feigning flight, halted in some well-known narrow passes, and there so destroyed the royal forces, thus unfortunately surrounded and entangled in the defiles, that the king himself, hiding among the slaughtered bodies, with difficulty made his escape on the departure of the enemy.
source 2, William Newburgh, Book Three, ch 6.3
There are also other examples, such as mongols and hussite, but you didn't specify context so I'll leave the classical "medieval" setting.
I hope that helps you out. :)