Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 21/03/2016
I first learned this choke at RGA Bucks from Matt Burn, back in 2010. It's become one of my main offensive options from side control since then, though I still don't have a good name for it. Gi tail choke will do for now, until I can come up with something better. It starts from the classic side control position, with an arm under the head.
Open up your gi with your far hand. In sparring, you'll sometimes find that your gi is already open, or you may need to be sneaky about it (e.g., from reverse scarf hold, so they can't see what you're doing with your gi). Punch that gi tail inside their arm, then feed the gi lapel to your other hand (that should still be under their head). Once you have the gi tail in place, get a firm grip: you may want to keep on feeding it further to make your grip even more secure.
Cinch it tight to their neck (keep in mind that it is the gi material that will choke them, so keep your fist out of the way), straightening the arm you have under their head. Put your free hand on the floor by their same side hip, to stop them following you (always a good idea if you are transitioning to north-south). Keeping your upper body low, walk your legs around towards their head, as if you were going to north south. At the same time, move your head towards their near hip: they will probably tap before you get there, but if not, keep going until you can put your head next to their hip.
If the submission still isn't happening, make sure you're keeping your arm straight and pressed into their neck, so that your gi lapel digs into the other side of their neck. There's also the option of raising your hips and jamming your knee next to their head to increase leverage. I tend to avoid that, as I don't like to give them any space, but it's an option.
Should you have difficult straightening out your arm, try moving your head further down their body, past their leg. That should enable you to stretch out your body further, giving you the room to straighten your arm out fully. It may also help if you turn your shoulder and straighten out your arm before you turn: adjusting mid-turn can be tricky. Finally, keep in mind that your grip should mean that you can return to a tight, safe side control at any point, then either attempt the same submission or go for something else.
Teaching Notes: I was reminded of John Will's great point about becoming a good observer, as teachers won't always show all the details they actually use to make a technique work, often because they don't realise they're doing it themselves. That happened to me tonight I think, as I've been turning my shoulder in before I begin to rotate for the submission, an adjustment that helped one student to get their arm in the right spot. Neither of us realised where he was going wrong until I demonstrated again on his training partner, at which point he saw what he needed. I'll be sure to remember that for next time: useful teaching pointer for me! ;)