Many avalanche transceivers can automatically "revert" from receiving to transmitting (i.e., searching to sending). The concept is, if you're buried by a secondary avalanche while searching, your transceiver should automatically switch back to transmitting so other rescuers can find you. The "Auto revert to transmit" row in the comparison table shows which transceivers have this feature.
Some avalanche transceivers base the decision to return to transmitting after a fixed number of minutes have passed without any user interaction, some transceivers won't revert when they are receiving a signal, some transceivers allow you to choose the number of minutes before reverting, some transceivers base the decision to revert to transmitting after a number of minutes without any significant movement (so they won't revert when you are searching, but will revert if you get burried while searching) and some use a combination of these considerations.
It is worth noting that transceivers that base the decision to revert on whether they are receiving a signal (which makes sense, because you might be on the surface and searching for your buried partner), will not return to transmitting mode if you are buried by a second avalanche and end up in a location where your transceiver is receiving a signal from another buried person.
A significant downside to automatically switching from searching to transmitting is that your transceiver will probably get ripped from your hands if a secondary avalanche does occur. Rescuers will then search for your unattached transceiver while you suffocate. We are not aware of cases where this actually happened, but it is worth considering.
Another significant downside of automatically switching to transmitting occurs when a rescuer's transceiver begins transmitting without the rescuer's knowledge. In this case, other rescuers may begin searching for the rescuer's transceiver.
BCA (the company that makes the Tracker transceivers) is concerned enough about the downsides of auto-reverting that they require you to enable this option each time you turn on your Tracker. Pieps is also not fond of the feature—they didn't add auto-reverting until the DSP Sport and DSP Pro, and the option can be enabled/disabled only at a Pieps service center (using a special cable). Interestingly, the automatic switching to transmit cannot be disabled on the Ortovox 3+ and Ortovox Zoom+. Due to its unique proximity switch (which changes between search and send modes), the Pieps Micro's auto-reverting cannot be disabled.
Pieps developed the Pieps Backup transmitter as an alternative to automatically switching to transmit mode. The Backup is intended to be worn in addition to a traditional avalanche transceiver. As long as the Backup doesn't sense a nearby transmitter and it does sense motion, it remains silent. If you are buried by a secondary avalanche and your transceiver is torn from your hands, the Pieps Backup (which will still be attached to you) will begin transmitting.
All transceivers that can automatically switch from searching to sending should give a loud warning before reverting so a rescuer doesn't unknowingly begin transmitting during a search (an unfortunately common occurrence during trainings). The Pieps DSP Sport and Pro are probably the best in this regard. Before automatically switching to send mode, they emit a European-siren sound for 30 seconds. After switching, they continually blink "AR" (i.e., Auto Revert) and continue to sound an alarm. The ARVA Axio emits only four beeps.
Another deadly situation can occur when rescuers who are not actively searching with their transceivers (e.g., they are probing, shoveling, etc) turn their transceivers off so they won't make unnecessary noise. In this case the transceivers are firmly attached to the rescuers, but they are turned off. In the event of a secondary avalanche, these transceivers will not be transmitting.
To avoid this situation, the Pulse, ARVA Axio, and ARVA Pro W transceivers have a special "rescue send" (or "standby") mode. When in this mode, the transceiver is not transmitting (which would confuse other rescuers) and it isn't receiving signals (so it remains quiet). The transceiver will only return to transmitting mode if it remains motionless for several minutes. Most manufactures suggest that this feature is intended for professional rescuers. The downside may be remembering how to activate this mode.
To enter "rescue send" mode on the Pulse:
>>When enabled, "Rescue-((SEND))" will be displayed on the screen.
>>When enabled, if the Pulse is nearly-motionless for four minutes, it will automatically revert to transmitting. (Before reverting, the Pulse will give numerous long beeps and ask you to confirm the change. Pressing any button will extend the "rescue send" mode.)