Why, during a magneto check, does manifold pressure increase as RPM decreases?
On the intake side, the engine is effectively a vacuum pump; it's sucking in air past a restriction in a duct - the venturi and throttle plate, creating a partial vacuum in the intake passage, and measured as total pressure in inches of mercury (In cars, the MP gauge is labelled &quot;vacuum&quot;; similar, but expressed as a differential between ambient and duct pressure instead.  The problem with car style vacuum gauges is you can't use it to measure horsepower because you don't know the absolute pressure in the intake duct, only the differential, and you'd have to know the actual ambient pressure to work out what is the actual intake duct pressure).
With the restriction being fixed (not moving the throttle), anything that changes the pumping rate (directly tied to piston speed, therefore RPM) affects the amount of suction created between throttle plate and cylinder.  Decrease the pumping rate (or sucking rate you might say), and the vacuum goes down/MP goes up.  And vice versa.
So anything that raises RPM without moving the throttle plate moves MP down a bit because the piston is sucking harder because it's going faster, and anything the reduces RPM without moving the throttle plate moves MP up a bit, because the piston's moving slower.  As you get familiar with this, you learn to drop MP about half an inch below target when following up with an RPM reduction, knowing it will come up as you reduce RPM. And vice versa on power increases.