As Josh said, it modulates line pressure. Soft shifts at light throttle, firmer shifts with more throttle.
If you leave it un-hooked, full-power shifts will still be soft and burn up clutches faster. I suppose though, that you could pin the cable at full draw (i.e., at WOT). I don't know what driving would be like this way, but you shouldn't have to worry about burning it up. Light throttle shifts will likely be noticibly firmer.
--Dan