Although ‘Alley-oop’ sounds like something a frenchman would utter following the untimely passing of wind, it’s actually a term that describes a spin in the opposite direction to your overall direction of movement. Essentially, an alley-oop in a halfpipe is when the direction of your spin is back up the pipe.
Take a look at Peetu Piiroinen’s backside one here. Backside alley oops feel fairly similar to backside 360s on kickers, so you should have a decent one under your belt before attempting these. Once again, the trick is to not go too straight up the pipe but to take the kind of line you’d take for a normal frontside air.
Rather than popping back of the wall, try to project your momentum up out of the pipe and rotate your head and shoulders to initiate the spin. You won’t need all that much rotation for these bad boys.