
They are behind you with hooks in. Protect your neck first, then slide to the mat and turn in.
Having someone on your back is the most dangerous spot in jiu-jitsu. They have their hooks in, a seat belt grip across your chest, and a direct line to your neck. In competition it gives them 4 points, and the rear naked choke ends a lot of rolls.
Protect your neck first: hands up, chin tucked, fighting their choking arm. Then slide down to put your shoulders on the mat on the side of their bottom arm, clear the top hook and turn into them. Never turn the wrong way into their choking arm.