I'm not sure exactly....the directions are a bit vague about the
penalties for not rinsing. Perhaps it is remnants of the chemical
reaction that need to be cleaned out, rather than the chemical itself.
I do have this mental images of it eating through my foundation,
though. ;-)

the acid wash
leaves a chalky residue when it dries if your painting over concrete
you need to remove this residue so the paint will bond to the concrete.
(I am assuming your going to paint)

Seems as if it ought to leave salt. (Calcium chloride?)