I've not heard it described in precisely those terms but what I assume it means is when under heavy load the motor slips out of synchronisation with the ESC. This usually happens during fast acceleration of the motor and/or when a sudden high load is applied. Basically the motor cant produce enough torque to 'keep up' with the rotating magnetic field generated by the ESC, so it slips back thus losing synchronisation with the ESC.
Personally I've seen this more on planes than on helis, it's quite common on 3D planes that spin big props. It's usually quite obvious when it happens because the motor emits a loud squealing noise, I've often heard it called 'timing squeal'. If you do experience it reduce the power immediately because the motor will burn out very quickly.
Often running more advanced timing in the ESC fixes it, if not you have to prop down.