if you ask me, i would think you would want to advance the timing as rpm increases. the faster the piston is moving, the sooner you want the spark. otherwise it would fire closer to tdc which would not allow for maximum use of the combusted fuel. you want to time the explosion so that it occurs just as the piston reaches the point just past tdc. so in order to achieve that at a faster rpm, you need the spark sooner. retarding the timing would cause spark at a later time which would mean the piston may be on the downstroke when the explosion occurs. it seems like it would push it down faster, but wouldnt you want that push from the maximum point in which the piston has just started its down stroke? which should be 1 degree atdc.