Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 27/01/2016
The situation for this is that they have stood up in your closed guard. As they stand up, if you've got a grip on their collar or head, maintain it in order to keep their posture bent forwards. At the moment you let go of that grip (if you have one) and they try to reach an upright position, grab behind their ankles (around the outside: if you grab around the inside, there's an injury risk).
Open your guard (when they stand, they are looking to open it and pass. It's better if when you open your guard, it's on your terms rather than theirs), bringing your knees together under their chest. You can also put your feet on their hips, depending on their height and how much leverage you need. Either way, drive those feet or knees into them. That should knock them over if they aren't prepared for the sweep. One advantage of the knees is you can keep squeezing your legs into their sides, which can help you use their momentum as they fall back (but be careful you don't get your feet under them too much, or you might hurt yourself as you hit the floor).
After they've hit the mat, before they can react, come up on your hand and same side knee. Bring your hips forward on that same side. It's much easier if you move in a diagonal direction, rather than trying to go straight forward. Slide your knee on that side to the mat, keeping your hips low, also grabbing behind their head (or collar). From there, you could go to mount, s-mount, side control etc. It is an awkward position, so takes a bit of getting used to. I used a hip thrust drill during the warm-up to help: you can do a technical stand-up from here too if you find that easier, keeping hold of their leg and passing around to the side.
Teaching Notes: I tried adding in a drill to help pushing through to mount, emphasising how you drive your hips forward at an angle. I have tended to tell people to keep a hand on the foot to help stop them sitting up before you can drive forward. However, I wonder if it is more helpful to instead put the hand out further forwards, to help that drive. I will try switching to that to see if it helps: once I've got some kind of drill sorted for this, could perhaps help with coming up from the tripod, sickle and handstand too, although then again those end up in slightly different leg and arm positions.
Something else I should note is the importance of including some breakfall drills when teaching something like this, as you're getting knocked over. To the extent I should probably put it in the main chunk of the write-up, as a reminder to me too.