Move your hand to the outside, so your pinky is around the locking collar, which is on the inboard side.  Then rock your hand counterclockwise, pressing down on the grip with the knuckle of your index finger to hold the throttle still.  Then hold the collar between pinky and thumb, and flick it into the engaged position by rotating it in the throttle-on direction. The collar will spring into the grip when it is rotated far enough.