[What's_nitro?]

To elaborate a bit-

Slotted motors have the stator coils wrapped through steel laminations (similar to a brushed motor) and the magnets on the rotor get caught at precise points where the poles of the rotor magnets align with the poles of the steel laminations (determined by the # of poles on each).

Slotless motors do not have the steel laminations in the stator coils so there is nowhere for the rotor magnet's poles to get caught on as it turns and so it rotates smoothly by hand.