This 12-volt probe tester does more than the 721 testers. The 721 has an indicator light that turns on when the sharp probes are pushed into a flat tape run that has 12 volts running through it. The 721-1 probe tester has a similar light but also a buzzer that sounds when 12 volts are present. It also can detect if you have an AC or DC circuit and if it is DC will indicate which leg of the tape run is positive.
There are two red LEDs: one behind each pin probe. Push the two pins into the tape run: One pin into each copper run. If 12 volts are present, one or both LEDs come on and the buzzer will sound. If one LED lights--you have 12 volts DC present and the probe in front of the lighted LED is touching the positive lead. If two LEDs are lighted--you have 12 volts AC. In both cases the buzzer will buzz indicating 12 volts present.
Be sure to check out probe tester 721 in comparison to 721-1