1. Assume any kind of prone position
2. Switch to third person view for better perspective of your avatar
3. Move mouse around, left and right, as fast as you can.
You can see how absolutely ridiculous this looks and feels, when body of your avatar rotates around with a speed of helicopter's blades. Yes, ARMA already features such behavior in standing and crouching positions, but at least in those it's theoretically possible to turn around this quick, so we can write this off to ARMA still being a game and some approximations are inevitable.
It's absolutely not possible in prone positions, especially when encumbrance from backpacks and other equipment is taken into account, and it should be treated seriously because of great amount of tactical advantage these positions give you, as mentioned above.