Underarm collar choke. I'm getting this from what we call the S-mount. Notice how my foot is planted here. My knee is behind his neck. I'm going to take the top collar, and I'm going to take it through his throat, under his throat, and pass it to my hand coning around his neck like so. One thumb in, four fingers out. Like so.
Now my hand here is going to go across the back of his neck. It's going under this arm, under this armpit, across, and you get the choke. S-mount here. Knee behind his head. Take the collar. Feed it to your other hand, and get it nice and tight. This is loose. It's not the same thing. You have to dig in, as small as possible, going in that direction. Now my hand goes under, and I have the palm facing me once again. I just walk this hand in that direction. Like so. That's your choke.