A backside shifty is one of those really useful skills that supports your normal riding and a feeds nicely into your freestyle.
If we think of a snowboarder comprised of two distinct areas separated by their belly button then we have the lower half which is your hips, legs and feet (board) and the upper half which is your torso, arms and head.
As a rider these two areas need to communicate and work in unison to ride a board fluidly or perform certain tricks. Yet, for some movements or tricks these two areas need to work independently. Thats where a lot of people struggle. The idea of separation, i.e rotating your lower body independently of your upper body is tough skill for many people to grasp.
With a Backside Shifty you're aiming for your shoulders to remain pretty much pointing down the slope as you twist through your hips and rotate your lower half to face downhill. The more you relax and keep your shoulders in line with the fall line (downhill) the more twist you'll be able to achieve with your lower body. Looks at the photos above and notice how my shoulders are pretty much still following the fall line.
The more you open your chest and face down the slope the less lower body twist you'll be able to achieve and you'll be tightening up your lower back.
To get a proper idea of the movement think about kicking a football behind you. Pay attention to how your back arm moves to assist kicking your back foot. They move in opposite directions. Think about that movement on a board. In essence your back arm/hand is by your back pocket and as your hips rotate your back hand moves towards your front knee. It might take a bit of time to get used to the movement. Try it off the board first, maybe on a trampoline. That way you can experiment with the movement in the air.
Try and remain compact when you're in the air. You can see in this picture that my legs are up as if I wanted to grab the board. This keeps my centre of mass over the board I feel more stable and in control.
Quite often riders will try to shifty with straight legs and end up throwing themselves off balance. Think of the sequence as pop/ollie, suck legs up, rotate hips into the shifty and then rotate hips/board back underneath you for the landing.
Once you have these on lock it can be a great kicker warm up move that will set you apart from the crowd and also act as the final stage prior to a Method which as we all know if the one of the most iconic snowboard tricks.