when you burn up the fluid you burn out all the friction additives in the ATF fluid. Which in turn changes its coefficient of friction. Since ATF fluid these days is sooo slippery the clutch packs have very tight clearances. When you burn up the fluid it causes the clutch packs to bind and lock when they arent supposed to. So it is perfectly acceptable for a tranny flush and fill to fix the problem. Though not always the case, it seems like he got lucky on this one.