  We develop a variant of the generalized slow roll approach for calculating
the curvature power spectrum that is well-suited for order unity deviations in
power caused by sharp features in the inflaton potential. As an example, we
show that predictions for a step function potential, which has been proposed to
explain order unity glitches in the CMB temperature power spectrum at
multipoles l=20-40, are accurate at the percent level. Our analysis shows that
to good approximation there is a single source function that is responsible for
observable features and that this function is simply related to the local slope
and curvature of the inflaton potential. These properties should make the
generalized slow roll approximation useful for inflation-model independent
studies of features, both large and small, in the observable power spectra.
