This is a way to throw someone to the ground with their horse. To throw both horse and rider to the ground is done in this way. When you fight on horseback, ride on his right side. Throw your right arm over the neck of his horse, and take the bridle by the bit which is in the horses mouth, and forcefully pull it up and over. Let the chest of your horse push through the rump of his horse. And in this way he will fall to the ground along with his horse.
When fighting against a mounted opponent, one of the options available to you is to take out your opponents horse. This has already been explored by the 1st counter to the 5th master of horse and the counter to the 6th master of horse. In this play you are unarmed, so here you throw both horse and rider to the ground.
For the purposes of explaining what is happening here, this play does not in any way take into account what your opponent is doing. Unless they are highly distracted, such as from dealing with another combatant, they wil be trying to fight you off. You will have to make this play in whatever space you can find while fending off their attacks. Given that there is just too much randomness going on there, it is described as used against a fairly passive opponent. Also, although there is a degree of movement between the horses, they are not moving at nearly the speed used in other plays from horseback.
Reach over the opponents horses head and grab the bit right at the mouth. Lift up and over the horses head, pulling back towards your self as shown. This gives an axial twist down the horses neck and into the shoulders, destabilising them.
Keep your own horse moving forward, pushing against the rump of your opponents horse. This causes it to move the back legs, pivoting on its already unsteady front legs. As you do so, push your right hand forward and down as if trying to touch your opponents saddle.
By adding a lateral twist to the axial twist you have aleady put on the neck, while disrupting its rear legs, the horse will be left trying to pivot with all its weight on its front left foot while you push against it. An impossible task, it will fall away from you, hopefully injuring the rider as it does so.
The horse will be angry, scared and probably in a degree of pain, but essentially uninjured. It will thrash around quite wildly as it gets back to its feet and runs off. Bear in mind that its feet will be facing you as it does this. Keep well clear of it while it does this before either making your escape or riding down your opponent down.