Just go and look at the bottom of the stem. Better yet, take the stem off and it will all make sense. It's more a spacer than a shim. The bottom (which the stem sits on) has an offset cut. So in one position it will angle the stem up 2*, etc. And if you turn it 180* it will angle it down by 2*, etc.

But like homebrew says, if you want to be more stretched out (ie. longitudinally) rather than more drop, you need a longer stem.