It is all in your feet and how you shift your weight. You should start with your right heel off the floor and pointed towards your opponent. Drop it flat on the floor, toe at a right angle to your target. Twist your hip and shoulder as far to the right as possible without moving your left fist. Let the resulting tension whip your left fist to your right shoulder, your left palm facing the floor at to end.
You can also do this by sliding your ight foot to the right (the shorter the move the quicker the punch. The further the move the harder the punch) dropping it flat and proceeding as above. Check out Al Hostak for a better description of this move- he used it to KO Freddie Steele in 1 to win the MW crown in 1938. Also, James Toney vs Robinson and Hernandez (?) the Puerto Rican Cruiser that went 12 with Hearns.