Yeah blowing air onto it. The way it works, well the stock hsf anyway, is the heatsink absorbs the heat from the cpu, and the cpufan blows air from atop itself into the little grooves and stuff of the heatsink, blowing the air away totally.
But honestly, would it be the same if the direction was reversed? I always thought that the hot air would get recycled and an uneven air pattern would be formed making things hotter, or only the top of the heatsink would be cool whereas the rest of it would still be hot.
BTW, not a stupid question