Translation
This scholar wants to throw the player from his horse by grabbing the stirrup and lifting up. If he does not fall to the ground, he will surely be in the air. Unless he is tied to the horse, this play cannot fail. And if he does not set his foot in the stirrup, grab him by the ankle, which works just as well in the same manner as I just described.
Explanation
For this play to work, there needs to be relatively little movement between the horses. When the direction of the horses are opposed to each other, as drawn, the context would most reasonably be the crush of a melee where the horses are wheeling around each other.
It is worth noting that neither combatant is armed in the picture. If you try this against an armed opponent, they will need to be highly distracted, as your head and back will be fully exposed at the crucial point of the play. While holding the reins and stabilising yourself by holding onto the saddle with your left hand, reach down with your right hand to grab your opponents ankle or stirrup. Due to your exposure, this is the hardest and most dangerous part of the play. Keep your eye on your opponent at all times.
As you sit up, you will be able to lift your opponents foot over the horses withers. Your opponent will roll back in the saddle causing them to fall to the opposite side of their horse. If you attempt to pull the foot behind your opponent, the throw will not work.
As a slight variant on this play, it is also possible to perform this play when both horses are moving in the same direction. You will need to be moving slightly faster than your opponent so as to able to lift the leg forward.