Puts the radio to sleep. Every so many milliseconds it wakes up and if there is a signal, it does what it always does. I don't believe this mode has any effect if you are scanning.
If it's the same as previous Unidens models it works like this...
Battery Save - works when there is no transmission for 1 minute in Scan Hold Mode and any Search Hold Mode (without Priority Scan). This feature turns off RF power for 1 second and turns on it for 300ms to extend the battery life.