Clinton, you are varying the prop between low drag (coarse pitch) and high drag (fine pitch). You describe the high drag situation as particularly noticeable.
The only way to get the drag down to (effectively) zero would be to feather it, but the single engine (piston) CSU does not have that ability.
When the mixture goes to ICO, the prop RPM will tend to decrease. The CSU, doing its job, will see the RPM drop and send the props more fine, in an effort to bring the RPM back up to "on-speed".
The CSU doesn't know the engine has failed; all it knows is that the the prop is now in an "underspeed" condition.
But the props hit the fine pitch stops - and the RPM does not come back to on-speed (unless you are in a dive, with plenty of airspeed).
If the prop RPM is still within the governing range however, you can command a more coarse - and lower drag - setting by pulling back the RPM lever.
If however, your airspeed is too low, then the prop RPM will be below the governing range
but on the fine pitch stops thus generating the high drag.
At high RPM (full fine) the propeller may have a negative angle of attack, and is acting as a big braking disc.
Only if the torque is low enough to result in the CSU commanding the fine blade angles.
This is exactly what T28D then said - but note the different terminology he uses:
One must be really careful with any (geared) engine not to allow the propellor to drive the engine, i.e. maintain positive manifold pressure and enough blade angle to always load the engine ...