Interesting Observation

I was playing with a bunch of motors on the bench last night, seeing
which ones would run off my 20V .5A power supply.
I observed an interesting difference between my servos and the
regular DC motor. If you grab the shaft of a vanilla DC motor and stop
it you can feel the shaft pushing against your grip, trying to go.
If you do the same thing with the servo, you feel a definate cogging
when the shaft speed gets to zero. It's like the torque "switched
off". If you let go of the shaft, the servo starts spinning again. I
am guessing that this has something to do with the servo's ability to
"hold" postion.
Anyone want to enlighten me as to why? I still don't understand the
difference.