Quote Originally Posted by Kraxis
What I have notice is that the target has to be weak, in open formation (as most archers and skirmishers are), of low mass and it has to stand its ground, or at least be quite slow in running away. The cavalry on the other hand can be pretty much whatever you have at hand.

Thus I have never experienced this against real infantry, only archers and skirmishers, not even Peasants (could be their horde formations that ruins that).
That is what I've seen as well. It is a case of the cav running through the formation. To do that requires a formation that isn't too deep or too dense (a horde formation is deep, although sparse.) The cav formation seems to get enough separation on the backside to continue the charge without pause.