  We study the dynamics of two-field models of inflation characterized by a
hierarchy of masses between curvature and isocurvature modes. When the
hierarchy is large, a low energy effective field theory (EFT) exists in which
only curvature modes participate in the dynamics of perturbations. In this EFT
heavy fields continue to have a significant role in the low energy dynamics, as
their interaction with curvature modes reduces their speed of sound whenever
the multi-field trajectory is subject to a sharp turn in target space. Here we
analyze under which general conditions this EFT remains a reliable description
for the linear evolution of curvature modes. We find that the main condition
consists on demanding that the rate of change of the turn's angular velocity
stays suppressed with respect to the masses of heavy modes. This adiabaticity
condition allows the EFT to accurately describe a large variety of situations
in which the multi-field trajectory is subject to sharp turns. To test this, we
analyze several models with turns and show that, indeed, the power spectra
obtained for both the original two-field theory and its single-field EFT are
identical when the adiabaticity condition is satisfied. In particular, when
turns are sharp and sudden, they are found to generate large features in the
power spectrum, accurately reproduced by the EFT.
