Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 16/01/2017
The first guard pass most people learn is the single underhook, sometimes known as a smash pass (although confusingly, there is also a completely different pass you might see called the 'smash pass'. The joys of BJJ's non-standardised terminology). After you've opened their guard (this can also work off a failed armbar or triangle attempt on their part), you need to get one of your arms under their leg. Your other elbow – and this is absolutely key – must not slip in front of their other knee. If it does, then you're at risk of being triangled: they simply need to pull the arm forwards to move into a triangle set up, as your first arm is already out of the picture.
You don't want to leave that first arm under their leg, as unless you're much bigger, their leg is always going to be able to outpower your arm. Therefore you need to get their leg up onto your shoulder, either bumping it up with your arm, or dropping down to put your shoulder in place behind their knee. At that point, drive forward so that you're shoving their knee into their face. When you've got them stacked, reach your stacking side arm around their leg and grab their collar. I tend to go four fingers in, but a thumb in grip sets you up for a simple (if somewhat crappy, so it's mainly for distraction) forearm choke. You can also try grabbing their opposite shoulder.
Establish a wide base with your feet, pushing off your toes. As is generally the case with jiu jitsu, stay off your knees. Otherwise, you're transferring the pressure into the floor rather than into your partner. Keep on driving forward, turning the shoulder you have behind the leg downwards. Combined with your forwards pressure, that should slide their leg out of the way.
Although it's tempting, try to avoid lifting your head to get past their legs, as that could provide them with space. Instead, you want to rely on your weight and pressure, finishing with that slight shift of your shoulder. To further enhance your stack, you can grab the back of their trousers, or alternatively put your other knee there as a wedge.
Teaching Notes: Things to emphasise next time would be really driving their knee towards their face, as a few people were going sideways. Also, somebody asked during drilling about dropping your knee. I think they probably meant turning the hip, as you wouldn't want to put your knee right on the ground. I included the kneeling break in this class, but I think that caused confusion. It's probably better to have the kneeling break in it's own class, perhaps combined with the standing break? Otherwise people get stuck on breaking the guard open, particularly when there is some resistance added in.
Also, from what Matt H said at a later open mat, you could try the Faria option where you don't come around so much. Instead, Faria likes to very slightly lift his head and gradually knock their leg out of the way, even pulling it with his hand. The idea is to avoid them having the chance to block with their arm into the hip. Worth a try, though it runs counter to a central part of how I normally teach this (turning and driving through the shoulder). Something to play with in sparring, at least. :)