Constant MAP assumes constant power. With increasing forward airspeed
the blade section angle of attack reduces. If the blade angle remains unchanged
the RPM would therefore increase. To maintain the RPM the blade angle (pitch)
must be increased thus increasing the torque and slowing the propeller to its
original set RPM.
For your information fine pitch is associated with high RPM and low
forward airspeed. Coarse pitch with low RPM and high forward
airspeed. Blade angle of attack is determined by RPM and TAS.