To perform an Auto Zero, spin the crankset backwards 5 complete revolutions. After the fifth revolution the power meter will automatically reset the zero point. If your power meter features an LED light, the light will flash once completed.
The ADVANTAGE of performing an Auto Zero is it does not require you to get off the bike. It can be done on the bike while coasting.
The DISADVANTAGE of this method is that it will will not report a Zero Offset value. Monitoring the behavior of the zero offset value is a great way to insure the power meter is functioning properly.