several EQs do this now...the massive passive for example, and GML
with the shelf switch in, the bandwith knob controls the dip right before the boost (or boost, right before the cut.
there are several uses for this...like any other 'not a trick'. it's just a thing.
if you have to cut lows on a track, a small boost right before the cut can make
the results sound less thin, for example.
likewise a boosting low shelf can be cleaned up with a dip on the top of it.
similar thoughts on the high band.
i have often thought as far as passive EQs go, if there is any trick...
it is using inductors not gyrators, as gyrators don't seem to saturate.
but i am just another guy buyin' donuts...
YMMV...'cause you are on a different road.