This is the counter to the previous play of lance and sword. The man with the lance strikes his enemies horse, because the swordsman cannot beat the lance aside when it is held so low.
The 6th master of horse carries his sword in posta dente di zenghiaro, so as to beat your sword aside before it strikes him. Counter this by dropping your lance low and driving it into the face of your opponents horse. Your lance will be well out of reach of your opponents sword at all times.
The horse will probably be killed outright, collapsing to the ground in an ungainly, thrashing heap. Even if it lives, it will be blind will pain, rage and panic and will be completely uncontrollable. Your opponent will almost certainly be thrown to the ground, possibly hurt and unarmed, and struggling to get clear of the dying horse. Take advantage of the chaos to ride them down before they can recover.