A very over simplified and basic answer, pistons were a long time ago!
When RPM is reduced for a constant throttle position, the rate of Inlet and Exhaust valve openings is reduced, thus, the engine cannot process the incoming Fuel/Air mix so quickly, nor can it dispose of the exhaust gases so quickly. Thus, the inlet pressure (Manifold Pressure) "backs up", leading to an increase in pressure.
Not so for mechanical superchargers however, lower engine RPM means lower supercharger RPM, which means less Fuel/Air mixture delivered to the cylinders. (Don't know about Turbo chargers, never had the privelage!
).
Pistons were 33 years ago, standing by for much more current operator's responses
Regards,
Old Smokey