When you twist a grip backwards you are putting your wrist at a 90° angle in relation to your forearm. You won't have any adverse effects from that alone unless you like to ride like that. The problem is, if you nose the ski in, all of your weight gets put on your hands/wrists and when your wrist is bent that severely and you put a ton of force on it, you're just asking to break it. The other problem I see is your grip isn't stationary so if you land with a lot of weight on your hands, there is nothing stopping your wrist from twisting the grip which goes right back to the first scenario.