If you get to the point where he has already rolled through (you are on your back), post both hands on his hips to keep him from being able to walk his lower body towards you to tighten up the choke. Then bridge slightly and scoot up so that your back is on his chest. Then, you can hook his arm (not the one that is around your neck...the one that he has his hand on your back....in the pic from Aesopian's post above it would be the guys left arm) by hooking your arm on the same side over the elbow (basically putting his tricep in your armpit and locking it down with your elbow). Begin to turn in to face him, and you can start to set up an Americana on that arm. Hope this makes sense.