Artemis BJJ (MYGYM Bristol), Can Sönmez, Bristol, UK - 13/07/2016
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 added in more drills for getting up after the sweep, as that's the part most people have trouble with. Basing out with a hand helps. Ideally, the hand would be grabbing their ankle, but that does make it tougher to get up. I'll keep playing with the drills, if I can come up with something useful, that would be applicable to all sorts of sweeps. :)