My husband says that is the most basic attack (he calls it thermo-rectal cryptanalysis; which is when they stick a soldering iron in your ass and then turn it on and wait until you tell them everything that you keep secret before they turn it off) and is just one example of why physical access is often game over.

Another involves liquid nitrogen and stealing keys out of RAM.

Another involves voltage manipulation to get chips to leak keys and why you want only public keys in certain chips (like the BIOS which is just read-only flash memory that has an electrical switch for enabling writing) He likes Libreboot/coreboot.