The rshl, or reverse short-hop laser, is simply an shl performed in the opposite direction that you're running / facing. It's an invaluable technique for retreats and maximizing your mobility.
Performing the rshl is almost exactly the same as the standard shl. The only difference is that you need to use the 'reverse neutral B' technique. Once you're in the air from the short hop, tap the analog stick very quickly in the opposite direction you're facing, and then shoot / fastfall like normal. It takes practice because it is very easy to hold the analog stick too long and try to shoot while it is still being held in a direction, resulting in a side-B rather than a laser. Just keep practicing this technique (try to find a stage with flat ground and something to obstruct any accidental side-Bs) and naturally through practice you'll acquire the timing and will perfect the rshl.
Hope this helps and good luck.