"Loading the backup RouterBOOT loader
Hold this button before applying power, release after three seconds since powering, to load backup boot loader. This might be necessary if the device is not operating because of a failed RouterBOOT upgrade. When you have started the device with the backup loader, you can either set RouterOS to force backup loader in the RouterBOARD settings or have a chance to reinstall the failed RouterBOOT from a ".fwf" file (total 3 seconds)"