What's this security mechanism called?

[GWiens2001]

As far as which key to trap, I am not very familiar with that kind of lock, but I suspect it works the same way. The wrong key will not keep the key pins at the correct height, allowing the trap pin to lock the plug.

Gordon

[Evan]

That's interesting.

But, how does the lock know to trap a certain key? Also, how does it knows if the lock has been picked or if a key has been used?

@minifhncc:

As far as how the magic works on trapping a certain key, that is one function I was not able to disassemble and figure out... However, the Best SFIC that was used to trap a key for the electrical rooms at one of my former employers worked exactly the way I stated, the GM and ENG selective master keys were unaffected, when the missing change key was used by one of the contractor's employees who had not returned it, it was "collected" by the core which was then removed by a co-worker by destructive methods on a day I happened to not be at work so I was unable to examine the specimen before it was destroyed to recover the key and then the remainders thrown away...

~~ Evan

[rphillips52]

As we're talking about what are, really, 'detector locks', it's worth mentioning the first detector lock was invented by Thomas Ruxton (1816). Unfortunately he often made his lock in such a manner that the false key was trapped, and there was then no means of resetting the lock and it had to be opened destructively. It is not still made.
Chubb's detector lock (originally 1818) could be reset, and versions were made until recently. The original used a separate 'regulating' key (Chubb's word) to reset. This was found to be inconvenient and the design was modified to allow the working key also to reset ('regulate') the lock. The lock signalled to the keyholder that it had been tampered with, as by an attempt at picking or use of a wrong key, but then could continue to be used. Numerous detector locks were used throughout the 19- early 20th centuries, when surreptitious opening was perceived to be a greater threat than forcible opening. Post WW2, that perception reversed and detector locks largely went out of favour; cost was also a factor.
And now we are back to lock cylinders which can only be opened destructively after a picking attempt ... . Such is progress.