Originally Posted by cyccommute

Look at your picture again. If you forgot to fasten the strap, it could easily unwrap and hang down enough to catch the spokes of the wheel. The wheel will come to a dead stop in very short order since there is very little stretch in the strap. Your buckle can easily catch a spoke and has enough strength to stop the wheel. Hook a part of the buckle over one of the spokes and test it your self.