It seems to be limited to horses running/charging, as I've not seen it in infantry. I noticed the horses do the combat leap animation, so it's like they've come into contact with the enemy - but you know, if you were to try and leap over a forest of 15 foot sharp pointy sticks, the results would generally be fatal (interestingly enough though, none of the infantry died from having 10 dead horses falling on them).

Anyway, I'm now quite the expert at the phalanx on/off command - in fact, I generally leave most troops with phalanx off until the last possible minute to benefit from their faster movement. Yes, I'm into micro-managing... OK, I'll admit, I'm playing with the speed reduction mod as well, so these tactics may only be advisable/practical for all you speed demons out there.

Just don't all come running to me when you've developed RSI and arthritis.