[Peter Ebbesen]

To actually get troops like cavalry to attack the rear and slaughter commanders, I've most commonly found it useful to deploy a solid infantry screen across the entire front as close to the enemy as possible set to attack closest except the infantry groups on each flank, which are set to attack rear. The cavalry is then positioned on the flank about halfway from the centre to the front with the order to hold and attack rear. [And you do have your few astral mages placed in the cavalry starting position casting body ethereal, right? At least for important battles]

When it works, this means that your centremost infantry locks down the enemy's infantry core for a few rounds while your flanking infantry envelops them, thus giving a free path for your cavalry - when your cavalry then charges, even if it wants to deviate from the rear, it has to move past the enveloping infantry to get at the enemy, which means that nine times out of ten it does end up in the rear with the lance attack ready to slaughter commanders, bodyguards, or the occasional unfortunate archer contingent. When it doesn't work, you'll often still end up losing less cavalry in general since your infantry eats that first round of damage and your cavalry gets to charge somebody already engaged.

This tactic depends on having numerical superiority in infantry (though inferior infantry quality is fine, so long as they can survive for a few rounds) or an enemy who isn't prepared for it - it is easily defended against by somebody who holds back substantial infantry groups screening archers/leaders on the rear-flank with hold-and-attack as they'll move to intercept the cavalry.

...Of course, if your enemy does start to hold back significant forces from the front lines because he's wary of you locking down his forces in melee and sending in the cavalry it just might be worth a consideration to try another setup with the cavalry as an iron fist punching straight through the centre of his weakened front lines with your infantry playing backup rather than the other way around. (Easy way to get badly mauled if he depends on crossbows, though)