Reading your sig, if that's what it's called:
"Teaching is the one profession that creates all other professions"
I figured you would have shared a link to that explanation!
here ya go:
How Does It Work?
As dirty air is drawn into two-stage air cleaners, it is forced to spin. This spinning action causes the dirt particles to be thrown to the outside of the air cleaner where it accumulates in the dust cup or the vacuator tube. This action is known as centrifugal pre-cleaning and is an effective method of removing much of the dirt from the incoming air before it reaches the filter.
When the engine is running, there will be a slight vacuum in the air cleaner. This vacuum will cause the vac valve to close, preventing air and dirt from entering. However, the engine will also have a pulsing effect on this vacuum, causing the vac valve to open and close slightly and rapidly, expelling the accumulated dust from inside the air cleaner. The vac valve will also expel dust when the engine is stopped, so the vac valve makes the pre-cleaning action of the air cleaner ‘self cleaning’.