I believe the tools intended job is to simply face and square the crown race but it is used to cut it down. I think this may be a pricey tool though.
here is some info on the VAR tools for this
http://www.sheldonbrown.com/var/pages/var0024.html
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk