I've seen several ways of finishing the arm triangle/side choke. Figure-four the arms like a RNC and grab their forehead or the back of your head. Sitting out like kesa to finish. Hips high for kata gatame in judo. Doing it from mount, half guard top and bottom, bottom guard, etc. I've even seen it from UNDER side control. But I never really liked any of these finishes. They all seemed to rely too much on arm strength and often crank the neck as much if not more than they make someone pass out (especially the RNC grip, grabbing their forehead and sitting out).
Then a fellow named Jeff Rockwell posted this explantion of how Aurelio choked out Gomi (registration required):
This is by far the best finish I've ever seen or done. I pulled it off on a ~340 lbs. wrestler last Friday, and we were both amazed. I would never have been able to even close my arms around his neck if I figure-foured them, and I would have just been rolled if I switched my base. I've been testing it out on everyone since then, and they've all commented on how they start going out as soon as I put weight on my shoulder. My instructor was even heckling me for always bringing in "Internet Jiu-Jitsu", until I showed it to him and he told me to teach it to a guy who had come to him with the problem of having arms too short to finish the arm triangle like a RNC. He's even adopted this finish over the one he's been doing for years, like Jean-Jacques taught him.
Some more pointers I picked up on other forums:
The way Shaolin finishes the side choke/arm triangle is by completely sprawling his hips flat to the ground, then circling his body towards his opponent's head. This tightens the choke without having to apply as much arm strength. He also uses a palm to palm grip with his head all the way down to the mat. I have been having MUCH more success finishing this choke since I started doing it this way. Make sure you really drive your bicep tight against his neck, and that your forehead is all the way down on the mat, trapping the back of his arm. Hope this helps.And...
-- Jeff Rockwell
Its a great move that can be done from top half guard(even when he tries a kimura), from side, from guard, from mount and even when you have his back and he tries to escape(see eddie vs leo vierra).Enjoy.
Few more details. The arm has to go far enough around his head so that your elbow touches the ground. The purpose of circling is so that you can create a screwdriving-like pressure. You can see the circling in some wallid, bustamante or mino fights. Shaolin also circles but not as much. Experiment on a basketball to feel the pressure you can create.
This is one of my favorite moves. I have been using it regularily in my sparing for the last year and a half. But, to get good at using it on top you have to have good transitioning skills. Your arms are in use so you cant use them to help pass the guard or some other transition.
Look up shaolins fights with hansen and nakayama, I am sure you can find them on the net. You will learn alot by watching him.