This is a sneaky foot lock from side control when your opponent puts their leg up to protect from getting mounted. Perfect opportunity to snag :). In this scenario, you will set them up by applying uncomfortable pressure on their throat, shifting your knees towards their hips, lifting a leg up, wrapping your arm around their ankle, then falling to your side with a foot on his hip for escape prevention. To finish, apply pressure to the back of his achilles with the blade of your forearm and lean back.
Notice the Leg Drag position? Foot locks all daily!