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The control loop/drive circuit for hobby servos is very simple. The input control pulse ranges from 1ms to 2ms with a typical duty cycle of 20ms. The rising edge of the input control pulse fires a monostable in the servo whose pulse duration is controlled by the position feedback pot (1ms to 2ms over the range of rotation). The output pulse from this monostable is differenced (actually XOR'd with polarity steering) with the input control pulse to produce a drive pulse that is sent to the motor. The greater the difference between the input control pulse duration and the monostable pulse duration (controlled by the position feedback pot), the longer the drive pulse resulting in greater power delivered to the motor to drive it back to the desired position. This forms a proportional position servo loop.
To answer your question, if you discontinue the input control pulse, the monostable is never triggered and no drive pulse goes to the motor. The motor can then be backdriven through the geartrain by the load. If the load is not significant, the output shaft will not move.
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