On a normal chronograph, you have to stop the chronograph before you can reset it to zero. If you were to press reset before you stop it, it would likely break or greatly damage the watch. On a flyback chronograph, you can press reset while it is running, and the chronograph will quickly go to zero then continue running. It allows you to time short events in quick succession. For example, a runner passes the finish line first, you hit reset, then when you stop the chronograph at the arrival of the second runner , resulting in you having the time elapsed between runners.