The 12V Force Charge Mode is automatically activated under certain circumstances and does not require manual selection.
In order to optimize your experience, the HULKMAN team set the battery voltage range (0.3–2V) for the 12V Forced Charge Mode during product development. When your battery voltage is in the range 0.3–2V, the 12V Force Charge Mode will be activated automatically, and the icon will start flashing on the screen. You just need to press the "Confirm" button to enter the Force Charge Mode.
Click on this link for details about the 12V Force Charge Mode: About the 12V Force Charge Mode of Sigma 1 battery charger