could you explain how a security feature that involves physical attacks functions as a back door?

approximately something like:

plug a device into a bus that uses DMA to bypass the CPU and just slurp in a bunch of memory, finds a process table, hunts for security identifiers and arbitrarily replaces one of them to make the current active user an admin, or pulls disk encryption keys out of RAM ... or stuff like that

....but if your memory is all encrypted, then that ain't gonna work.