The switch contains a lot of hardware fuses, with an instruction to blow one. IIRC it's used (among other things) to set a minimum firmware revision - after up

The switch contains a lot of hardware fuses, with an instruction to blow one. IIRC it's used (among other things) to set a minimum firmware revision - after upgrading to a new (signed) firmware it blows a fuse so you can no longer run an older signed firmware, which stops you rolling back to when a vulnerability still existed.