yup.. high sync speed will enable you to set a shutter speed that is faster than your camera's sync speed. Check your manual on how to activate it and note that you probably wont be able to bounce your flash if you use HSS..
TS is asking about why there is a limit to the fastest shutter speed when the on-board flash is turned on. it's a hardware issue, whereby the on-board flash isn't powerful enough to function at higher shutter speeds. unless, of course, TS goes for an accessory flash with X-sync capabilities