That is what I was trying to find out.
But why is it only used 91/92 and why not just use one way check valves that were used in previous years?
Never had a Mitsu T-1, but after looking at the vac diagrams, this is very clever engineering by the Chryco folks. The use of check valves would not create an area of pressure differential, which is what is needed for the creation of a Venturi effect.
In a set-up like my TII car, the cruise function recovers in an abrupt, unrefined, manner once the vac system is exposed to boost. Since the cruise servo works off of vacuum only, and the brake booster has a finite ability to store vacuum, the servo can lose the ability to control vehicle speed until vacuum pressure builds back up in the brake booster (the primary vacuum source for most TII cars). Resuming a higher, saved, speed can be sluggish as the car wants to go into boost to do so; but under boost, the cruise servo doesn't see enough vacuum to actuate the throttle as aggressively as your right foot can. One way to alleviate this condition: install a vacuum reservoir downstream of a one-way check valve leading to the brake booster so that you effectively add a back-up vac source inline with the cruise servo.
An easier way is this nifty vacuum ejector arrangement, which effectively eliminates the need for a separate vacuum reservoir. Pretty slick. If you look at my modified diagram, and assuming you understand the Venturi effect which is a result of the Bernoulli Principle, then it is very easy to see how vacuum is created in the line to the cruise servo under boost. The Ejector is actually like a Venturi nozzle - if they labeled it as such in the first place, the concept would be more readily evident.