The Countdown mode in the Auto Refresh Plus extension for Chrome is a feature that allows you to set a countdown timer that counts down from a specified time interval to zero. When the countdown reaches zero, the extension will refresh the page.

To use Countdown mode, you will need to follow these steps:

1. Click the Auto Refresh Plus icon in the Chrome toolbar.
2. Click the “Options” button.
3. In the options page, locate the “Countdown” label and turn on the “Enable Countdown Mode” toggle.
4. When you next open the extension’s toolbar popup, you will see a tab labeled “Countdown.”
5. In the “Countdown” tab, you will see a field where you can enter the time interval that you want the countdown timer to start at. For example, if you want the countdown timer to start at 10 seconds, you would enter “10” in this field. You can also specify a specific date and time for the countdown to reach zero.

When Countdown mode is active, you will see the countdown timer displayed in the extension’s toolbar popup. When the countdown reaches zero, the extension will refresh the page.