Translation
I am the ninth and I am doing the counter to the counter before me. When he turns his sword, I immediately do as you see drawn here, so that I cannot be struck with the pommel in the face. And if I raise the sword upward and give it a reverse turn, you can well see that the sword will be taken from you. And if I do not do that, I will give you a backhand strike with the blade or the pommel will strike you in the head, I would turn it so much. Here ends the plays of sword against sword on horseback. Who knows more about it would give a good lesson.
Interpretation
Against an attack, your opponent has made the play of the counter to the 8th master. They have beaten your attack aside and then keeping the momentum going, have turned their sword fully around to make a counterstrike to your face with the pommel.
Note that in this picture, the player who makes the counter is wearing a garter. The scholar who makes the contra counter wears no insignia.
As your opponent makes their pommel strike, knock it aside with the handle of your own sword, as shown. Keep your sword upright and the handle braced against your forearm as much as possible. Keep your elbow tight to your centreline. It should feel like you are striking the pommel aside with the base of your hand. Roll your wrist as you make contact, so as to further throw the pommel offline.
Raise your sword above your head, then drop the tip to horizontal. You will have already begun the clockwise motion with the roll of your wrist. If you have caught your opponents sword handle right at the base of their hand, lifting the sword and dropping the tip like this will lever the sword out of their hand.
You have two options to continue with, depending on your distance. You can keep your hand as a fixed point, and make a horizontal cut with the true edge of the blade into the right side of your opponents head. If the movement of the horses has brought you too close to make a cut, raise your elbow to face height and keeping that as a fixed point, make a pommel strike instead. It will feel like making a riverso dagger strike. the pommel strike is slightly slower, but targets at a shorter range.