Quote Originally Posted by Puzz3D
This is possible in the game, but you have to substitute another unit to take the place of the unit that is disengaging because the AI pursues units that try to disengage. In actual battles where men could stop fighting and withdraw, I suspect it's because the enemy didn't pursue.
Sure, I expect the complete disengagement was a mutual thing as Bob the Insane says, but being able to fight while retreating is possible, if prone to collapsing into a rout should the enemy force the pace. I don't think very compact units like pike phalanxes could pull it off though. Having also done a bit of re-enactment shieldwall stuff, I agree with Grumfoss that even fairly minimally trained troops are capable of a slow retreat while facing the enemy and still maintaining order.
Possibly if your army charged down a hill and failed to break the enemy, this would be a way to pull back and regain your advantageous position without having to turn your troops around. Similarly, it could come in useful in bridge battles.
There is a small element of this already in MTW, units can get pushed back during combat, especially downhill, but I think this is a more organised concept. Probably too complex for the games designers to bother with, I guess.