When you're sailing upwind efficiency = speed = height so you set the mast rotation to maximise the lift to drag ratio. When you turn downwind, power becomes king and efficiency is less important. Rotating the mast will increase the available power. Also note that the main is eased a little its not cranked hard like upwind.

The good guys (Bundy etc) play the main downwind but you have to be very confident in what you're doing because we mere mortals have a tendancy to dump main when we get in trouble and as noted above dumping main can equal dumping mast.