This is another counter of lance against sword, where the one with the lance sets and holds his lance under his left arm, so that his lance will not be beaten aside. In this way he will strike with his lance against the swordsman.
The concept of this play is an exercise in leverage. Although shown against an opponent chambering their weapon in Posta di Donna Sinistra, this play would work equally well against anyone with a sword on horseback. Regardless of your opponents guard, they will be aiming to beat the point of your lance aside before closing.
Striking the head of the lance like this creates a first class lever. The hand acts as the fulcrum, and as the sword strikes the lance head, the butt of the lance wants to swing out to the right. If you were to couch the lance under your right arm, as is usual, the lance is only as stable as your capacity to hold your arm tight to your body. Bracing the lance under your left arm is slightly awkward and reduces the lances manoeuverability, but it is locked in place by your entire body weight and your grip in the saddle. The lance wil break sooner than be beaten offline.