Suuposedly you should try and 'Roll' i.e , keep your hands tucked in. If you stick them out (which is quite instictive), there's a chance you can break your collarbone(not a good thing). Of course, this assumes you know your'e falling and can't do anything about. I think some people (mainly racing types), practice falling, so they get in habit
of doing this. Make me glad I don't race....