I was reading a book on LDs by Stephen LaBerge the other day and he reckons that it's the feeling of motion that solidifies the dream (which is why spinning works). So stuff like falling backwards works as well although a lot of them result in false awakenings. To battle that he suggests that while spinning you keep repeating "The next scene I see will be a dream" and so even if you do have a false awakening you can identify it as one and remain lucid.