The button is on the back of the EasyFlash, next to the lights. While you hold it, the blue and red lights flash.
A short press approves the Bluetooth pairing the EasyFlash is waiting on, or the next one within a minute. See Bluetooth and Wi-Fi.
Hold the button for 5 seconds and the EasyFlash restarts into its factory firmware. The factory firmware only installs the full firmware again: it doesn't talk to vehicles and has no Bluetooth or Wi-Fi. Use it if the EasyFlash stops working after a firmware update.
To get the full firmware back:
Don't hold the button during a flash from the EasyTune App. Restarting the EasyFlash stops the flash part way through. When the EasyFlash runs a job by itself, it ignores a long press while the job is writing to the ECU.