Emergency Trigger
This just came up recently on an LAFD thread at
http://radioreference.com/forums/showthread.php?t=45699, so I'll just steal part of that and add a little:
If an officer needs help but can't grab his radio to ask for it, they can push a button on top of their portable radio, which automatically switches the radio to the "trigger" frequency and signals the dispatcher that he/she needs help and identifies the particular radio.
The officer gets immediate access to an RTO (dispatcher) who's not busy, on a frequency where nobody else will be talking over him/her. The RTO gets the display of his unit number, and his last known location, and broadcasts "All units officer needs help, 1-Adam-12's ROVER has been activated, 1-Adam-12 what is you location?" (or "1A12 shows on family dispute at 1234 W 40th Street...")."
R.O.V.E.R. was the acronym LAPD and Motorola came up with when radios with the help trigger were introduced in L.A. in the late 70s... Remote Out of Vehicle Emergency Radio.