Basically spool is how fast your turbo builds boost, some turbos spool faster because that is how they are set up. Which means they hit the boost you have them set for sooner. The bigger the turbo usually the slower it will spool as an example, thus you have to have a different stall set up for your converter to get the turbo to build boost quicker. That's why people are always referring to combinations, certain turbo's need a certain size converter or you don't have the spooling you would like. I hope this helps. I'm sure others will have their version to define. Hopefully..