You mean building rebound into the shock so the shaft extends on its own after being compressed (sans spring of course) right?

From my experience, you want little to no rebound on tracks that don't have 40' triples or quad jumps and is not rutted up (indoor) as the car will stick to the ground a little better and feel more settled.

But if you have large landings to deal with a little rebound in the shock will give you better bottom out resistance and the wheels will extend back down to help absorb the next rut (or crater on a blown out track) in the track (outdoor).

Building with rebound leaves more oil volume in the shock so it is naturally trying to push the piston down compared to no rebound where you push the shaft in and it stays in (no spring)... since you screwed the shock cap down with the shaft all the way in purging more fluid than building with rebound holding the shaft some amount out of the body leaving more fluid in than a no rebound build.

Maybe Fred or others have something to add to that or even a correction but that's what I've got for ya