Flingwing, don't you mean MP goes up when RPM goes down, for a fixed throttle position?
MrEdd, what you describe makes sense to me.
As FW said, the engine is slowing down due to one mag being turned off.
The slower engine is now sucking in air at a slower rate.
This allows pressure in the manifold to become closer to the ambient pressure.
As FW suggests in his example:in the extreme case that the engine were to stop altogether at that throttle setting, the MAP guage would read ambient pressure.
Interesting question, and I'm going to look for this effect at my very next Mag Check.
Cheers
WHK4