Toyota prius Owner's Manual page 212

2-1. Driving procedures
 Changing "POWER" switch modes
Modes can be changed by pressing the "POWER" switch with the
brake pedal released. (The mode changes each time the switch is
pressed.)
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PRIUS_OM_OM47871E_(EE)
Off
The emergency flashers can
be used.
ACCESSORY mode
Some electrical components
such as the audio system can
be used.
The "POWER" switch indicator
turns amber.
ON mode
All electrical components can
be used.
The "POWER" switch indicator
turns amber.