Opponent is upside your back, he to over turn your body by insert his arm from your armpit.
You grip his right arm, seize his right arm with your left armpit, in this case,
you seize more upper than his elbow.
With seize his arm, you head thrust into his right armpit, your back head lift his armpit,
you turn left and change opposite direction, you enforce his joint.
You keep seizing his arm, prevent his rolling escape.
You should keep his arm away from opponent's body.