Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 13/03/2017
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. Alternatively, you might manage to pull their shoulder up if you have a grip ready for the choke, whereupon you can slide your knee under and begin the switch to s mount.
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, tightly coming under their far shoulder. 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. Lean forwards, sliding 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 (which is why Saulo recommends leaning forwards). Make sure you don't flop backwards: it should be a slide down the arm, staying upright. You only drop back when everything else is tight.
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. Watch out for them trying to either push your leg off their head, or push the other leg down where they can trap it with their own legs.
Generally, you want to get both legs over them, bringing the heels in tight to their head and armpit. Crossing the feet tends to be a mistake, but that's because most beginners will relax their thighs when they cross their feet, making it easy for your partner to push your legs off. If you remember to tense your thighs, driving your knees out, crossing the feet can be a strong control. Be warned it takes some finesse though.
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 Notes: Getting that hip turn is something to emphasise next time, to make sure people are facing the other arm. I'll have to think of the best way to explain that: face the other arm might work, so try that, particularly if I also emphasise grabbing that other arm to both help maintain the position and hinder their escape. I could also talk about the figure four grip on the arm as a method of control, although that fits best if I've recently taught the kimura from north south. So, maybe if I ever teach armbar at the end of mount month, followed by a kimura at the start of side control month.