Actually holding your breath does not make you run faster, in fact if it does anything it will probably slow you down. By holding your breath less oxygen is pumped around your body causing build ups of lactic fluid. Holding your breath will lessen your endurance but will not affect whether you run faster or not. I am an extremely fast runner and I know from experience that running while holding my breath will result in me going at a normal or slower than normal pace and me having to stop constantly to catch my breath. Trust me, if you want to go faster, just practice that's all you need to do. Of course sometimes practice won't work, it's a personal thing. Sometimes all you need to do is burn of a little fat and put some muscle into those legs.