JTAG is a protocol that the manufacturer has customized according to the provisions of the agreement at the time of production. We usually do not recognize or access the USB or SD card until it has been started and loaded. When the underlying startup data of the phone is lost, the driver code cannot be loaded normally, and it cannot identify USB, etc., of course, it cannot brush the machine. At this time, JTAG can force the CPU to write the underlying data to restore the normal startup state.