i am guessing you mean when you rev it up then let off, as the revs come down, they dive below idle?
my car does this (RB20DET), reason mine does it is because it runs off the TPS and a MAP sensor, the vacum in the manifold is different when you i let off, and the ECU sets fuel and spark according to that, but once it gets down to a certain point it will come back up.
if yours is doing something different, then this means nothing. or if you are still using AFMs it should not do this for my reason.