Three things sit between your files and the connector. The controller is a small processor. The firmware is the program it runs, kept mostly in a service area on the flash. The map, the flash translation layer, is the firmware's record of where every logical block physically lives, and it changes constantly as the drive shuffles data for wear levelling. Any of the three can fail, and each fails differently.
Firmware faults are the commonest and the most recoverable. A page of the service area goes bad, or a write to it is interrupted, and the controller either stays silent or drops into its ROM. In the diagnostic state the bench gives the controller a working copy of its firmware in RAM, reads the service area around the damage, and rebuilds the map. Nothing is written to the flash, so nothing is lost by trying.
Controller faults are rarer. A chip that has overheated, a short after a surge, a crack after a knock. Where the controller is dead the flash is read directly: each chip removed, its pages dumped, and the controller's scrambling, interleaving across dies, XOR patterns and error correction undone in software before the map can even be looked for. That takes longer, which is what moves a figure within its band, and it is impossible on a drive that encrypts in hardware, because the flash alone is ciphertext. On those, Samsung, WD, SK hynix and Apple among them, the original controller is revived instead, which usually can be done.