Moon Walk is a technique exclusive to Super Metroid that can be toggled on or off in the Options menu. Moon Walk allows Samus to fire while walking backwards. For this to work, she must be charging or holding the trigger if the Charge Beam is disabled in the Samus Screen.
"MOON WALK (shooting while walking backwards)
When this is set ON, you can fire beams while walking backwards by pressing the Control Pad opposite to the direction which Samus is facing."
"To do the Moonwalk, simply hold down the Shot Button to fire up the Charge Beam and press and hold the Control Pad in the direction opposite of the way Samus is facing. This option is helpful because it allows you to move back and forth without having to take your gun off your target."