Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 20/01/2017
For this fundamental attack from the guard, you first need to get control of their arm. There are numerous ways to do this, but in the interests of keeping things simple for drilling, a straightforward option is to grab their opposite tricep with your hand, then pull that across your body. You're then going to put your same side foot on their hip, clamping the knee of that leg behind their shoulder (essentially you're trying to take away their space, as well as blocking them from easily pulling their arm backwards).
If they're wearing a gi, grab their opposite collar with your free hand (keeping a firm hold of their arm with your other hand) and pull them down. If it's nogi, grab their head. Next, kick your free leg into their armpit, aiming to further break their posture and get your leg across their back. You're also going to use that to swivel your own body away from their trapped arm and get a better angle. From here, you can then push their head out of the way with your head/collar grip.
That should make it easier to bring your hip-pushing leg over their head. Slide the arm you're using to control their arm up towards their wrist. At this point, you can switch to grasping their wrist with your hand if necessary. Squeeze your knees together, lift your hips and pull down gradually on their wrist for the tap.
A common problem is that your partner will 'stack' you up onto your shoulders, making it difficult (though not impossible) to finish the technique. This is a common problem with the triangle too. To prevent that situation, push with your legs, as well as really knocking your partner's posture when you kick across with the armpit leg. You can also 'walk' back on your shoulders to recover a more extended position if they are squashing you. Finally, angling the leg you have by their head can help (like on Adam Adshead's old DVD), as that makes it tougher for them to push into you.
Teaching Notes: After teaching both the triangle and the armbar, this one appears to stick better. The main stumbling blocks are similar to the triangle, in that people often forget about head control and/or don't swivel around far enough. I've got a bunch of other variations I can teach (largely thanks to that great class Chris Haueter taught on armbar in Leuven last year), so I must remember to add those in too occasionally. This still feels like the central one to go for, though that might just be because it's the one I'm most familiar with.