This is the counter to the previous play. If someone takes you by the stirrup or the foot, quickly throw your arm around his neck, and in this way you can unhorse him. If you do this he goes to the ground without fail.
The 2nd scholar of grappling from horseback is attempting to throw you off your horse by grabbing your foot or strirrup and lifting it up. Although this is an effective method of throwing someone from their horse, it leaves the person making the throw exposed at their mechanically weakest moment. This is the moment you are taking advantage of when making the counter.
The lower your opponent can grab you, the more leverage they get for their throw, but the more exposed they are to a counter. As they go to grab, they will be leaning right forward and possibly a little out to the side. You are aiming to extend this movement beyond their tipping point.
Reach across with your right hand and grab on to the back of your opponents neck as shown. Try to catch their momentum as they are reaching out. Keep pushing their head down. It will help if you can move your own horse off to the side slightly. If you push straight down past the shoulder of their horse, they will be able to hold onto the saddle. If you pull them out to the side, they will have no support.
As with all throws, you will need to be quick and smooth, so as to keep your opponent off balance. Once past the tipping point, they will topple onto their head between the horses, to be trampled underfoot.