For this gainer dismount the gymnast takes off from a single front foot while the back foot swings forward helping to kick the knees and hips up to create the somersault rotation.  To assure that the skill travels away enough to clear the beam, during the take-off the athlete aims to bring her center of mass forward  so the dismount does not travel vertically.  During the last step, the arms perform an underswing to help set up the skill.  This young gymnast needs to work on her head position which is moving too far out, detracting from the tuck somersault technical lines.  In general, for all back tuck somersaults, the rotation is created at take off from the trunk area.  The head can be neutral or in some cases slightly out for visual purposes.  Throwing the head out does not have any rotational value.