If you shut the ignition off while riding at high speed, the engine will continue to rotate due to your momentum, but the engine will not be firing. Eventually the bike will just slow down and come to a stop because there is not spark to create the power in your engine cylinders. No spark means no fire, which translates to no power. This does not damage the motorcycle, it will simply turn off the ignition and the bike will slow down as it is no longer under power from the running engine.