Summary: The Pieps Backup is a one-of-a-kind device. Rather than transmitting all of the time, the Backup only transmits a signal if it does not sense a nearby transmitter and there isn't any significant motion. It's intended to be worn in addition to an avalanche transceiver. If you get buried (i.e., no motion) and your avalanche transceiver isn't transmitting (either because you were in search mode when you were buried and your transceiver doesn't have an "auto-revert to transmit" mode, or because your transceiver was damaged as you were strained through the trees), the Backup will begin transmitting.
One of the selling pitches for the Backup is that it's an effective way to replace the "revert to transmit" mode found in many transceivers. That mode causes your transceiver to return to transmit mode if you are buried while searching. Many avalanche beacons, including the Ortovox 3+, Ortovox S1+, Mammut Pulse, and Tracker3 have motion sensors that'll revert based on time and motion. Other transceivers "revert to transmit" solely based on time.
The downside of reverting if you are hit by a secondary avalanche while searching, is that your transceiver will likely get ripped from your hands. The Backup solves this problem.
The Backup's battery will last 150 hours in dormant mode and one hour in transmit mode. The single hour might be enough if you're going to be found alive (although 90% will be dead within an hour, 8% survive for more than three hours), but it's not very long for rescuers who might want to recover your body.
Remember that this is a transmitter and not a transceiver. You can't use the Backup to search for your partner.