Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 04/11/2015
Along with chokes, armbars are another good option from the back. You have the usual seatbelt grip, with one arm under their armpit, the other over their shoulder. Grasp their opposite wrist with your shoulder arm, then grab your own wrist with your armpit arm, locking on a figure four. Reach your foot on the armpit side over to the opposite hip, hooking around with your instep. Use that to swivel: you can also add in a swing with your other leg to help the rotation, much like with the bow and arrow choke.
You can also push off the floor if you prefer. As you turn, bring your shoulder arm over their head (this is often a fight, as they know they're in trouble once that is clear), then keeping their arm tight and your bum close to their shoulder, bring your leg over their head. Maintain a firm grip on the figure four throughout. It will also help you prevent them turning inwards, a common escape to the armbar.
Finally, adjust your position if necessary (e.g., scooting your hips in closer to their shoulder in order to prevent giving them any space), squeeze your knees then gradually drop back. Don't let go of the figure four until the last moment, moving up to the wrist. Raise your hips and pull down on the arm to finish. Make sure their thumb is pointing up (if it isn't, you can still finish the armbar, it's just a bit more awkward as you have to angle based on their elbow).
To add further control, you can put your leg higher on their head, making it more difficult for them to raise their head up. If they do manage to turn in towards you, you're in a good position to move straight into a triangle from guard. Quite often they will also link their hands together: there are many methods for breaking the grip, but one I like is simply kicking their grip apart (making sure you aren't giving up too much control in the process).
Teaching Notes: I've been combining this with the RNC as a beginner technique, which seems to work. It's worth emphasising that you should sit up first, as dropping back means they can stack you and position is lost. Getting your arm over is tough part. I find doing it early makes most sense, pushing off hip, but it is possible to try it later. Also, I mustn't forgett to mention squeezing knees, pull on wrist, basic stuff. The other question that always crops up with armbars is grip breaks. Normally the leg kick is enough, but if they grab their collar (as somebody did today), that's not so effective. The semi-circular swing is perhaps the best one to teach in that situation, and indeed the first grip break I learned anyway. ;)