Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 08/04/2016
For the standard technical mount choke, use your hand nearest their chest to scoop up their arms, also preventing them from digging an elbow under your leg. You need to get a hand under their top arm to grasp their nearest collar, then open it up. Your other hand curls around their neck, whereupon you can feed the collar for a tight grip.
Your first hand is now going to push through past the crook of their elbow, catching their arm in the process. Bring that hand behind their head, then pull your collar gripping hand back towards you, snaking around their neck. This should result in a tight choke: your first hand is mainly blocking their arm, rather than playing a major role in the choke. Simply keep driving that arm through as far as you can.
If they manage to block your choking hand, you can instead switch to an armbar. Your top arm will reach through and grab your own gi, while your other arm will post directly in front of their face. This help prevent them escaping the attack: if you want to be mean, you can also dig the elbow of your first arm into their side. Lean towards their feet, step your leg around their head, then drop back for the armbar, squeezing your knees as usual.
Teaching Notes: There is a Saulo choke version of this, where you grip differently, but I'll save that for later: it's harder to land, as you have to get the pressure right. Main thing I want to emphasise on this next time is what your pushing arm does. You keep extending it past their head, sliding behind. A few people seemed to get confused with the motion, so I'll spend a bit more time demonstrating that next time. All in all worked ok though, as it isn't a difficult choke. :)
I didn't go into detail on the armbar, but I'll leave that in the notes for next time. I did mention it, as that's a good follow up which is right then when you're going for the choke.