This is one of my favourite no gi attacks when I'm in side control if I can isolate the arm.
a visitor to our gym hit it on me about 4 times in one roll session so I decided to try to learn it. I studied up on it on Marcelo's site quite a bit.
The trick to making it work is being able to control your opponent all the way until you sink that shoulder down.
First when you collect the head, then block the hip with your other arm as you walk around to north south while gathering up the arm with your hip. Keep your hips down on the mat tight. Use your knee to scrape his arm off the mat if you need to.
Once the arm is tight to the head connect your grips in an S grip. You really need to keep the guy flat on his back and you need to get under the chin.
Marcelo teaches to use your rib cage to turn his head if it's not in position. You want the chin up and centered. Once you get there, I like to think of sliding down to stretch out the neck a bit. Like a crocodile dragging its prey into the water. This helps position the chin and cinch up the choking position.
Once you are there making sure you keep both your elbows on the mat I will pull slightly with the arm on the outside and roll your shoulder that's on the neck down towards where your hands would be under the neck. Don't concentrate on the squeeze, focus on dropping the shoulder. make sure you keep nice and flat with your legs wide and hips down.
theres a few variations but this is the one that works for me. I find it works well with my relatively short arms.
Marcelo even jumps into this position straight from the pass sometimes. I'm working on that part still.