You put one above, and one below the stem. You crank one taller, the other shorter. Bam, fast adjust, and you don't need to remove the topcap or anything. Mind you, seems like a great product for dialing in a bike, but I would way rather depend on aluminum (or hell, even carbon) spacers for real riding.

This is true actually, what pardus left out was the fact that making beer involves BOILING the spring water for an hour. ;)