sorry for the late response:
pdsp::Amp multiply the
in_signal() input (that is default for patching so you avoid calling it), for the
in_mod() input. Usually the signals goes to the first and modulations like anvelopes, lfo or other means of control to the second, the second inut is checked first and in case of a modulation being constant at a value of 0 all the signal path is not calculated.
@skywalkerwin n the code you posted a trigger control is patched to control an envelope, and that envelope is patched to control the amp modulation, probably another signal (like oscillator, samplers, etc) is patched into that amp somewhere else in the code, so the envelope controls the volume of those sources