🚀 go-pugleaf

> Take the case where you are charging down the road at 4,000 rpm and you
> let up on the throttle without depressing the clutch.  Under the above
> theory the turbo would continue spinning at tc(4000) since the engine is
> still at 4,000 rpm.  However the boost guage indicated an IMMEDIATE drop
> in boost.

Right you are.  A turbocharger is driven by a turbine that, in turn,
is driven by exhaust gases.  If you let up on the throttle, that
decreases exhaust pressure, which decreases turbine torque, which
decreases boost.

This phenomenon is a great source of extra complexity in turbocharged
airplanes, where, for example, if you put the manifold pressure at
the redline at the beginning of the takeoff roll, you may blow
your engine...