The manifold pressure is just an indirect measure of the drag on the rotor blades - the power required to turn the blades.
The reason for the reduction in power required is that the large change in inflow as you go through translational lift decreases the drag, and hence the power required to turn the blades.
There is also the ground vortex being overrun that causes the large change in handling at about the same time.
Do the transition to forward flight from a higher height, and you'll notice less effect on handling.