Problem is, half the time it doesn't........corrosion from sweat, rain and moisture from wet roads can weld the stem to the steerer tube. So a slight tap might have to be more like sharp raps or pounding with copious use of penetrant/lubricant to break up the corrosion....
I don't think that bike designers ever really thought of the long term consequences of putting dissimilar metal together tightly when they designed bikes and it's components.

Chombi