This was shamelessly ripped from the KX3 reflector, originally posted by N6KR and saved here for future reference:
Battery Operation: To conserve battery power, use 5.0 watts or less when running from a 12-V source, and 3.0 watts or less when running from an 8-10 V source (including internal batteries). At these levels, the KX3 automatically switches the power amplifier to a more efficient setting. You can tell that power-saving mode is in effect by a decimal point after the ‘W’ in the power value (e.g., “5.0 W.”). To reduce supply current in receive mode, use headphones rather than the internal speaker, and if lighting conditions permit, turn off the LCD backlight (Menu: BKLIGHT).
Oscillator Isolation: The KX3, like other radios with a quadrature direct-conversion architecture, uses a VFO (local oscillator) running at or very close to the operating frequency. If you have another radio on the same band as the KX3, and the antennas are close together, the other radio may be able to hear the KX3’s oscillator when they’re both tuned to the same frequency.
To prevent this, the KX3 includes an isolation amplifier that keeps the oscillator from radiating back through the mixer. Normally this amplifier is turned off to save about 15 mA of receive-mode current drain. To turn on the isolation amp, set MENU:RX ISO to ON. This has no effect on performance.