From what I know MAP is a way of measuring engine power with variable pitch propeller. Greater throttle setting - greater power - greater MAP on the gauge. It happens because MAP is in fact vacum pressure so opening the throttle gives greater fuel and air flow into the manifold, explained here: Why does manifold pressure increase with power?
But I also read somwehere that when we reduce rpm without touching the throttle, the MAP will increase, according to this topic:What happens to manifold pressure reading following a reduction in both throttle and RPM?
Reducing throttle decreases manifold pressure. Reducing rpm reduces suction, so the MAP should then increase?
The question is: why does reducing rpm increase MAP?