The purpose of this valve is to prevent an engine runaway. The valve
vents the line to the ALDA whenever (hopefully never) the turbo boost
exceeds 16 psi. The electric pressure sensor is on the top rear of the
intake manifold.

Defeating this safety system creates the possibility of engine
destruction if the turbo's wastegate (relief valve) fails to open at its
10 - 11 psi threshold and an instance of full power that drives the
turbo pressure above normal limits. The ALDA then would become so
pressurized that it would add substantially more fuel to the motor
creating more boost pressure and making it run faster and faster until BOOM!

The valve gets dirty and may leak; that creates flat acceleration when
the ALDA doesn't get the proper boost pressure. Or, the banjo fitting on
the rear of the intake manifold gets clogged with soot (cleaning it is
easily done with a straightened paper clip).

You can experience the effect of the ALDA by test driving the car
without the banjo connected to the intake. The motor will run fine but
acceleration will feel flat; the hissing noise is the turbo pressure
blowing out of the banjo hole.