The key to maintaining the back-mount while falling to the weak-side is a strong underhook (armpit) control. If you have deep and tight underhook control while falling to the weak-side, your opponent can not drive his shoulder blades to the mat and slide out. This is why, unlike most people, Ryron prefers to connect his hands with the over-hook arm on top of the underhook arm (seatbelt control). This allows him to "pull" his underhooking arm as deep as possible. If the opponent defends the arms by grabbing the top arm, he is ok with that because they set themselves up for the arm isolation (top leg).