VHF radios have dedicated "1/25", "1W/25W" or "Lo/Hi" button that toggles between Full (25 W) and Low (1 W) transmit power. When either VHF CH 15 or 17 are selected (guard channels either side of VHF CH 16), Low transmiter power is automatically selected, in which case dedicated button will have no effect. Low transmiter power shorten transmission range and also preserves battery capacity.
Practice on VHF radio simulator
A single press of the 1/25 key toggles between Full (25 watt) and Low (1 watts) transmiter power.
Selection of Low transmiter power and return to Full transmit power