Hey,
So I searched the tech reference section and nothing in there on how a rack spacer works so here is a nifty drawing I made in MS Paint to illustrate it...
The Red shows the teeth on the rack that aren't being used normally, then when the spacer is installed, they are being used.
The Yellow show where the rack spacer goes.
The Blue is the plastic (usually) washer that limits the rack travel normally if not a steering stop elsewhere.
An alignment will be required after the installation of any sort of rack spacer as you do effectively lengthen the rack and thus need the tie rods to be adjusted shorter to compensate.
Any comments, corrections, questions... go!
Cheers
Bryan