You’re doing a flying start so that you’re not burning all of your energy overcoming resting inertia. This allows you to apply more energy later in the acceleration curve, which should allow you to achieve a higher speed within the set distance than would otherwise have been achieved from a stationary start. Think of it as giving yourself an extra 10 meters to get going. For example, if you are using a 20m build up distance, when you reach the end of the build-up you should ideally be moving at about the speed you would be at 10m from a full effort stationary start. Then floor it for the second 20m segment.