  The Effective Field Theory (EFT) of Preheating with scalar fields, implies
three types of derivative couplings between the inflaton and the reheating
field. Two of these couplings lead to scales below which only one of the two
species appear as the low energy modes. In this paper, the variety of low
energy regimes in terms of the species they accommodate are explored by
studying the scales introduced by the derivative couplings and the dispersion
relations they lead to. It is noted that the EFT of two scalar fields can give
rise to non-trivial sound speed for both the inflation and reheating sector
even at scales where modes of both species propagate freely, suggesting the
presence of additional heavy fields. The regimes where one of the species
affects the dispersion relation of the other while not appearing as an
effective mode itself, are named as "Hidden Regimes" during preheating.
