Transition from side back against turtle, to full back mount.
From side back against turtle. The opponent is in turtle position. You are on their side, for example on their left. Your right arm is around the waist, but not reaching in deep where it could be trapped. Your left leg is out wide, posting for balance, your right foot is near theirs, and your right knee is also near their knee. The right knee is not on the ground, all weight is being lent onto your opponent.
From here establish a seat belt grip. The left hand reaches between the shoulder and the head, and the right goes under the opponents arm. The right arm (always the one coming from under the arm) grips over the left wrist. This way if the top grip is stripped, the left arm which has the potential to apply a choke is still in position. Once the grips are established move the right leg behind and tight on the opponent, and importantly bring the left next to the outside of the opponents left leg. The knees should be next to each other, to block the opponent basing their left leg out. With this grip roll over your and your opponents left shoulder at a 45° angle. During the roll, or ASAP after get your hooks in on their legs. If they ball up lean back and look up to stretch them out and make some more space for your legs. Make sure once the hooks are in you are leaning on the master side, that is with the arm that is over the shoulder closest to the ground.