Unit 1 and Unit 2 were riding on Lake Bowen when Unit 1 stalled and could not start.  Unit 2 turned back to them and as he approached and let off the throttle could not steer the PWC away and collided with Unit 1.  Operator of Unit 2 did not know that you cannot steer a PWC after letting off the throttle.  Unit 2 glided into Unit 1 at approx 10 to 20 mph.  Operator of Unit 1 was injured on the right thumb.