As they turn to face the legs i block their arm so they cannot turn back to face my head again.. and take the nearest ankle and throw it over my head as i launch my hips into the air.. its a roll over and a simple solution to this pass. using the pic below as a reference my left hand would go right behind his tricep close to the armpit. My right hand would grab his ankle of his trailing leg (left) and rotate it over my head as i launch my hips up.
But the thing is you can only do it as he turns.. if he has turned and his hips are on the ground.. your screwed... and then option #2 come sin to play.
If his hips are set then i move for this. I get up on my elbow on the same side his hips are on. My other hand goes to his shorts/pants right at the butt crack. Grab the pants.. and then as he pulls his leg out to pass half guard simply tug him forward from the butt crack grip and slide to his back. It sounds easy, and it really is.. YOU HAVE TO stay on your elbow though.. other wise you wont have the space needed to go to the back.
using the above pic as a reference. I would sit up on my right elbow. Left hand would be the one on the pants/shorts, Left elbow will be framed against his back to keep the distance as well.
Important - when playing this guard dont let the above position happen with your arm there. i move my left arm to his back to do the above mentioned sweeps. In the pic your are proned to getting kimura'd. When moving the arm to the back never let the guy close the space and force your arm up to your head. If so.. the above mentioned sweeps will be useless as you will be pinned with no real exit other than to let him pass and try to recover.
You can keep your elbows in his back in order to keep some space and stop him from locking your down.