I think Peter explained it well enough, but if not, I'll give it a shot.
When in stop position, the two lines are connected by the keyswitch valve, and
it doesn't matter which is which. In the run position, the shutoff valve side is
open to atmosphere and the vacuum source side is closed off. This allows air to
the shutoff valve so it can deactivate. Like Peter said, the line to the IP
should have a vac leak in the running position and the line to the vac source
should not.
Also, if somebody stuck a one way valve in the shutoff line it would stay shut
off.
Mitch.
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