I was wondering about the same thing. When I went airborne from my mtb a few weeks ago I was definitely unclipped from my spds, but have no recollection of doing so on purpose. I may have just done it automatically, maybe they released by themselves.
But I keep my spds (Shimano, forget the model) on pretty light tension, and have never been able to accidentally unclip just by pulling hard on them (which I do, quite often), so I always figured I had to do the sideways twist to get my foot out. I've had another, worn pair that unclipped while using them though - very scary!
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett