Thus it can be used to detect mains AC voltage without removing the insulation from the conductor. Just take it in the vicinity of the conductor and it would detect presence of AC voltage. If mains supply is available in the conductor, the electric field would be induced into the sensing probe. Thus display count advances rapidly from 0 to 9 and then repeats itself. This is the indication for presence of mains supply.
Display stops advancing when the unit is taken away from the mains carrying conductor.