I demonstrate what the scholar before me explained. I pull you onto the ground by the visor. And if I wanted to do that with grappling, this technique is better than the others that I know well.
In this play, you are making a solid effort to tear your opponents head off. The use of the visor to add leverage means this play had to be drawn in one of the armoured sections, either pollaxe or sword in armour. However, as Fiore points out, it also works well as a regular grappling technique.
In the narrow play, grab your opponents visor. You will maximise the leverage if you start with the thumbs down. As you lift the visor, curl your wrists and elbows in as close to the centreline as you can get them.
If your opponent has no visor, then find any leverage point you can, and grab with the thumbs up. This will vary somewhat depending on helmet design. On a bare head, put the two small fingers from each hand in the hollow at the base of the skull, and lever your thumbs off the cheekbones or eye sockets. Just be sure to get a good grip on something.
Keeping the weight on your front foot, drop your weight down. Stretch either leg out behind you as you do so. The picture shows the right, but it really makes no difference. As your weight drops, pull your elbows down to your hips, rolling your hands forward as you do so.
With great force, move your weight straight back, shifting all your weight from the front to the back foot. At the same time, slide your elbows and forearms past your ribs. Keep rolling your hands over so that they end up right in front of your belt buckle with the thumbs on top. This pulls your opponents head directly into your core, bending it back along the spine.
At the last moment, pivot 180 degrees on the balls of your feet. This adds a lateral twist to the break, multiplying the damage. You will also have more room to roll the hands over still further, bending the head back even more, if possible. There is no question that their neck will shatter completely.
Drop the lifeless body of your opponent at your feet.