How does a megohm meter work?

The megohm meter (or megger) passes the 500, 750, 1000 volts to the wire that needs to be checked. According to ohms law R=V/I, the current in the returned voltage is measured and divided by the resistance (inside the meter). So when the current is high (i.e not properly insulated or leaking), the resistance value will drop otherwise resistance will be high.