http://switchbrew.org/index.php?title=Fuses
Basically the way it works is this: certain firmware updates burn (set) fuses in a region of the efuse memory. If the firmware version on the Switch’s internal memory is
x then you’ll need
y fuses set for it to boot. You can always set more fuses, but can never unset them. Therefore, as firmware version
x increases, fuse count
y may increase as well, and will never decrease.
This allows the console to check whether its memory has been tampered with to attempt a downgrade: if a firmware is on the system that requires 4 fuses burnt (3.0.1 does), and through an eMMC backup or otherwise the console’s firmware is manually downgraded back to 3.0.0, which requires 3 fuses burnt, the Switch will check the fuse count while booting and see it’s 4 and not 3, and it will not boot, and will likely blow another fuse in order to allow Nintendo to detect why it’s not booting. that way if you send it in for service, they can send it right back to you saying you voided your warranty by tampering with the firmware.
There is no known way to bypass fuse checks without having full control of the boot procedure. The Xbox 360 uses eFuses the same way to prevent downgrades, and it has definitely done its job there — as far as I know it’s not possible to downgrade Xbox 360 firmware, and the later its kernel version is the more difficult it is to hack the system. The “reset glitch hack” on the Xbox 360 involves programmatically triggering a reset repeatedly on the processor while sending bad data to the data lines on it (very specific data, very precise timing, and so on) eventually “glitching” it out causing it to load the hacked data that bypasses protection. It’s possible an exploit like this can be found for the Tegra X1 that would be able to be used on the Switch, but I oversimplified it and glossed over how precise it is — we’d need to know a lot more about the inner workings of the Switch and its Tegra X1 bootloader and so on for it to even be worth investigating.
In short, you’re not going to find a way to unset these fuses without violating laws of physics, but with very, very extensive knowledge of critical internals it MAY be possible to skip them from being checked in the first place, or to prevent them from mattering.