The drive of the back leg in the round off directs the turn over of the round-off thus insuring the speed of your pass thereby enhancing the height of the third skill.
For this drive, strength is required in the hamstring and glute area. In addition to that a certain amount of flexibility of the hip flexor is required because there must be a straight line from the fingers to the toes of the back leg and the foot cannot be turned to the side. Doing leg drives with rubber bands around your ankles will develop this movement as well as exercises for your glutes, hamstring, core and shoulder/thoracic area.