How can I perform a tactical withdrawal?

[Connacht]

2) When you wish to create gaps in the enemy formations for your cavalry or flanking troops to exploit. This is what Philip II of Macedon (Alexander the Great's father) did in the battle of Chaeronea.
http://4.bp.blogspot.com/_KezhQ6waZT...h/testvdfg.jpg
http://3.bp.blogspot.com/_KezhQ6waZT...0-h/chaert.jpg

Sadly^2 this is hard to do in game because the AI doesn't always keep very stable formations and make units charge almost independently.
So most times you have to move your own troops in order to surround the enemy, rather than forcing him to get in a fake gap.