Here is a pic of the valve
It goes on the cc breather, which is normally a neutral or 2 way operation. it is a one way valve that allows pressure pulses out of the cc but does not allow regression of the pulses.
The bigger the pistons the more air is moved around in the cc, 3.6 has a bit of a problem, 3.8 much more so. These types of devices enhance the natural dry sump induced cc vacuum. Very common on race motors w/ big pistons, and high perf Harleys & Ducatis of course.
there are other methiods of generating cc vac, from mech pumps to the exhaust driven method used by the nasty car guys