"cab snow angel"
Hahaha that one cracked me up. We have all experienced that one!
First, let's talk about why this is probably happening on chop while you don't seem to have this issue on groomed terrain. Most likely this is an unconscious defense reaction to your riding environment. On really steep off piste terrain we all have this natural instinct to put our hands out in front of us because through millions of years of human evolution, we have instinctively learned to catch our falls with our hands. On some level when you start to push your comfort zone, you are mentally "expecting" to fall so this defense mechanism kicks in. This instinctive reaction is also the reason wrists are broken so frequently in our sport. So, don't feel bad that you might be struggling with this; you are fighting millions of years of human evolution!
Now, is it "bad"? Not so much "bad" as inefficient and counter productive. As Donutz said, for a heelside turn, this movement is actually an enhancement to board performance. Even if yor entire shoulders and torso don't rotate, this is still a rotary movement and a change in weight distribution over your board. Where this really bites you in the bum is when you are trying to execute an aggressive high performance topside turn on steep terrain in the gnar. Then this becomes a so called "counter rotation" More accurately it is a countered stance since typically this hand stays pretty stationary in this position. The effect of this is it adds a heelside rotary movement or force to your board while you are trying to make it go topside. You are simply working against yourself which slows down board response and makes you use more sheer muscle power to perform at required levels and this is fatiguing. The goal of good, fluid, smooth and stylish riding should always be to ride smarter not harder!
So! How do we teach ourselves to overcome instinct? Muscle memory is the key here; train your body to react with new instincts through deliberate repetitive training. Your goal is to make correct movements at the correct time. I am not a huge advocate of the AASI "stationary upper body" mindset. It has it's place yes, but too often it makes the rider ridgid and robotic; while technically correct it is not a very fun way to ride and I think it looks like shit from a steeze point of view.
I like to see a little upper body movement and in dynamic riding which is required for steep off piste terrain, it is imperative to use good, correcty timed rotary movements. So, for heelside, that back hand should move out over the toe edge of the board at the same time that the front hand should travel over the heel edge. When executing the toeside turn, your front hand should move out over the toe edge and that back hand should not just stay behind you but actively travel around behind you to be out over the heel edge of the board.
Think of your arms as a compas needle. The front arm moves to point in the direction you are turning to and the back hand will move in the opposite direction. Now, this seems totally contradictory to everything AASI teaches and in a way it is. With basic riding, people instinctively try to throw their board with these huge, clumsy, wild upper body rotations and they flail around like a whirling dervish. As coaches, we spend a lot of time breaking this habit to get people to use their lower body to steer their board and ride with a quiet upper body. When they progress to dynamic riding, we have to get them to again reincorporate good upper body rotary movement to enhance board performance.
Here is a great drill to explore the effect hand position really has on your board's performance. You will feel a bit retarded doing this but humor me. Pick a nice, wide, mellow, well groomed run that you and comfortably straight line safely. Point your board and get up to cruising speed and stay as static as you can like a statue( keep legs loose and bent in your good athletic stance though). Keep your shoulders perfectly aligned with the board and don't move them. Now, simply swing your arms like you would when walking; front arm forward, back arm backward them the opposite. Do this slowly and hold them a moment at their farthest swing point. Notice what your board is doing! It's turning in the direction you front hand swings isn't it?
That drill is great to let you experience the actual profound effect that simple hand position has on your board's performance. Mow, you add that with a subtle shoulder rotation AND good lower body movements and you have the makings for a very effective, smooth, efficient, powerful turn that is going to aggressively drive your board where you want it to go in the worst shit the mountain throws at you! Every time you make a toeside turn in these conditions, make a deliberate movement to swing that back arm behind you.
Caveat: this is for very dynamic riding in challenging terrain. This technique is innaproriate for good basic carving and mellow cruising using skidded and scarved turns where you really want the stable, aligned upper body!