This choke is applied from the North-South position. It's one of the first submissions I learned for NS. When I first learned it, it seemed like it left a lot of space for the victim to try to escape. However, after trying it on a few friends of mine with varying degrees of skill, I have to say it works pretty well. Give it a try and see if it works for you too.
I start off in side-mount.
I scoop up Naqib's left arm with my left arm, gripping my own right shoulder with my left hand. I then place my right hand, palm-down against the mat, against Naqib's right hip. This prevents him from turning together with me when I switch to north-south.
I spin my legs towards Naqib's head and my head towards his legs to end up in north-south. I keep my chest pushing down on his to keep him pinned.
I remove my hand from around Naqib's left arm and start feeding it through under the back of his neck, palm up. The act of trying to feed my hand behind his head serves to push my shoulder deeper into his chest, further pinning him.
Once my hand is in under his neck, I try to wriggle it through to get his neck as deep into the crook of my elbow as I can. I then sprawl and put all my weight on his neck, trying to touch the palm of my hand to my chest while keeping the back of my hand on the mat. I know my sprawl isn't too great in this picture, I didn't want him to fall asleep before the picture was taken. If he doesn't tap after a few seconds, just start walking your way down. In other words, move your body in the direction opposite to his feet, to expose his neck more to the choke.
Proceed to get revenge for his stupid poses in step 2.