  We investigate both the null energy condition (NEC) violating scenario and
the $c_T$-diminishing scenario for generating enhanced power spectrum of
primordial gravitational waves (GWs) during inflation, where $c_T$ is the
propagating speed of primordial GWs. Both of these two scenarios can be
realized stably with theories beyond Horndeski, hence can be uniformly
implemented within the framework of the effective field theory. We calculate
the power spectrum of primordial GWs by assuming that the inflationary Universe
undergoes three phases, where the violation of NEC or the diminishment of $c_T$
occurs in the intermediate phase. A template of the spectrum is given for the
NEC-violating scenario. We also discuss the underlying relation and discrepancy
between these two scenarios with a disformal transformation.
