It activates/deactivates a “Super-Low battery mode”. It changes the brightness, changes the Wi-FI status (turns it “ON/OFF”), changes the Bluetooth status (turns it “ON/OFF”), turn “ON/OFF” the sound and pause the music.
The first time you press the toggle button, the “Super-Low battery mode” gets activated and turns everything “OFF”. When you press the toggle a second time the, “Super-Low battery mode” deactivates and turns everything back “ON”.