This is the lowest hardware level. If your phone is completely dead (hard-bricked) and won't turn on, you must short two hardware points on the motherboard (Test Points) while plugging it into a PC. The computer will recognize it as "Huawei USB COM 1.0". This mode allows you to flash a bricked device without an unlocked bootloader.
The general approach (used by Kirin-Tool) involves: hisilicon kirin 710 flash tool