My understanding is when (usually WOT), after shifting to the next gear, the RPM's jump way up for a second or 2. Scared the heck out of me after buying mine and experiencing this. Rebuilt now and no more flare.
From someone who has experienced it...When my car would shift 2-3 instead of it being firm it kinda slips a little between the shifts. It's not for long...maybe like .5 seconds, but I'm sure it gets worse if you don't do anything about it. I had my trans freshened up and my 3rd gear clutches looked they had seen better days.