Increasing
speed reduces the angle of attack on the prop blades.
If you
had a fixed pitch prop this would mean that you lost thrust, and the faster you
went the less thrust you would have.
But you
have a constant speed prop, and as the angle of attack reduces the torque drag
reduces and the prop RPM begins to go up.
The CSU
senses this, and increases the blade angle to get the RPM stabilised again.
So you
settle down at the higher speed with increased blade angle.
Think Hurricane Mk I, fixed pitch wooden prop with a horrendously coarse pitch
for high speed.