This is a finishing move and it is a good grip to use to hold someone in such a way that it cannot be defended against. The counter is that the one being held goes back against a wall or other timber and turns in such a way so that the one who holds him breaks his head or back on the same wall or timber.
Here you make a high grab from behind. There are a couple of different ways to get behind your opponent. You can, of course, come from either side equally well depending on the circumstances. For the purposes of description, the play is written as manipulating your opponents right arm.
You can counter an attack of theirs with an elbow push. There are many examples of these throughout Fior di Battaglia. Within the grappling section is the counter to the 11th scholar. You could also, somewhat more proactively, grab at their front hand like the 5th master of dagger. This will pull them off balance, allowing you to step behind. Either way, you will turn your opponent in an anticlockwise direction.
Regardless of which method you use to turn them, shuffle in close with your right foot. Step through with your left foot so you are behind your opponent. As you step, swing your left arm under your opponents left arm. Slide your right arm under theirs and using both hands, push forward on the back of the skull, as shown in the picture.
Although pushing the head down, you also need to pull their weight back. You want them to fall in a sitting position with their legs out in front of them and their arms jammed out to the sides. Once your opponent starts to fall like this, kneel down one knee and lean forward into them. This will lock the whole body in place.
Be aware that to counter this, your opponent will be trying to stay on their feet and bend forward at the hips. If they succeed at this, there are several things they can do.
If they get their hips under yours, they can throw you forward over their shoulder. This is particularly the case if they are much bigger than you.
They can lift you off the ground enough to run you backwards into a solid obstacle.
If they are reasonably flexible, they can bend right forward while stepping back and to the side. This would allow them to slide under your arm. They would then be well placed to counter you with the throw of the 7th scholar of grappling.