Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 14/10/2015
You've managed to move into high mount and get underneath their elbows. Reach your arm under their opposite forearm, past the crook of their elbow. Grab around their arm, so that you're gripping the tricep. You're then going to move into s-mount, in a motion that has some similarities to the technical mount switch. Pull the arm you've gripped across, creating space to slide your knee forwards. If you need additional base, post your free hand by their head as your knee comes up, swivelling your torso to face their other arm.
Your other knee does not raise off the ground. Instead, you're sliding it along the ground, then twisting and curling it around your opponent's armpit. If you do raise that knee, you're at risk of leaving enough space for them to escape. Keep the knee low. Leaning forwards once in position may help too, to maintain your balance.
Lock the arm to your chest, or secure it by grasping your own collar. Scooping up their other arm can be useful here as well, if you can, also giving you the option of switching sides if you need to. Some people will grip their curling-leg ankle from here, reaching under their opponent's head (which has the additional advantage of cross-facing them). This will depend on your leg length and flexibility: you'll want to be comfortable with the position first, which is an awkward configuration.
Keep your legs squeezing into them, then lean sideways towards their stomach. This is to lighten your knee-leg, so you can bring that over their head. Slide down the arm you trapped at the start, staying close to their shoulder. From here you'll be looking to drop back for the armbar. However, that moment where you're bringing the leg over their head is also where you're at risk of giving them too much space.
To prevent them turning into you, continue to lean into them, backstopping their elbow with your body. They need their elbow to turn, so don't let them have it. Grabbing their leg will make that even tougher for them, though note you'll normally need to switch arms for that. They will also try to bring their head into play, aiming to get to their knees so they can start stacking you. Use your leg to push their head away, so they don't have the posture to recover a strong position.
Finally, you will most likely find that they clasp their hands together in some way. There are numerous options for breaking the grip. The simplest and most universal is, I think, bringing your leg into the crook of their elbow and pushing the grip loose, combining that push with a pull from your arms. It isn't foolproof, but it seems to be the one that works most often for me. Drop back, squeezing your knees, then pull down on their wrist and raise your hips for the finish.
Teaching & Sparring Notes: I put in too much detail, an old habit that sometimes creeps back if I'm not careful. Next time, pare it down! A common problem was people leaving space with their curling leg, because they were lifting it before bringing it around the armpit. Next time, I'll emphasise that should be more of a 'slide', then you curl, without raising the knee up into that semi-technical mount position.
I mentioned grabbing the leg under their head, though I'm not sure how useful that is unless you're super tall or long limbed (so, good one for Ruth, come to think of it). Posting on your hand for balance is something I want to add next time, keeping it close to the head. That goes for when you're dropping back too. Getting a grip on the leg makes a big difference too (for me at least), so I'll emphasise that more too. Adding more grip breaks is tempting, but that will overcomplicate: however, having a few more up my sleeve for drilling would be handy.
In sparring, I was finding that leg grab made all the difference. I also combined the armbar with the single arm/Saulo choke from Monday, which worked well for me. Having said that, with more experienced people the single arm choke feels a lot less stable. I'll have to force myself to go for it more often on those experienced grapplers, so I can see where it breaks down and how they escape/defend. There's the combo video Saulo did on BJJ Library too, which I'll give another watch.