there are bootloder is the body firmware
so, if the new firmware includes a new bootloader, when firmware background it modified it in the background.
and firmware old firmware(even with the old bum) due to the nature of the specific device may not flash the old bootloader, and leaves zilch — a new quitter can not(or will not) run the old body firmware.
In such a situation, well, when old slacker provides emergency flooded with correct body firmware(for example when clamping of the conditional keys selects from the root of the SD card the body of the firmware and injects the flash)
But there are variations when intentionally or stupidity of the developer, and may be specifically due to regionlock and other stuff a slacker does not provide for such emergency procedure — then the body goes on JTAG-firmware all flushes.
Behind this, we need to look at a particular model of iron, and understand ourselves in the situation, and the ways out of it
For example if you take htc t5353 and flash it alternative a quitter, you can lower or raise or even to shed the incorrect firmware only manage after flashing to do a full reset, and in case of failure to lift the body from the SD cart