Help me, I'm too stupid to figure this out, Decel Valve Options

[pbanders]

I have no idea of how this thing can do what it does. I understand WHAT it's supposed to do - limit the intake manifold vacuum during overrun conditions - I just can't see HOW it does it. I've never seen the inside of one.

The valve has three ports, "control" (the skinny one on the end), "manifold" (the fat one on the side), and "atmospheric" (the fat one on the other end). The control and manifold ports are both connected to the branch "tee" fitting on the intake manifold, so they see the same level of vacuum.

If I remove the decel valve and bench test it, when I pull a vacuum on the control port alone, at some vacuum level (which is adjustable), the valve opens and I can blow through the other ports. However, if I simulate what the valve sees in the engine - that is, the control port and the manifold port both at the same vacuum level, as I expected, the valve doesn't open, no matter what level of vacuum I pull. I expected this result, because there's no pressure differential across the ports to open the valve.

What am I missing here? Clearly, the valve works in a car. How?