A wire, a pin. Maybe even a clothespin. Some kind of mechanical trigger that has to be disconnected before you get to the guts of the thing.
10 seconds sound clip from the Longstreet (Ep. 6 Spell Legacy Like Death) TV Series.
You can hear this line at 00:17:34 in the DVD version of the movie.
Quote context
[...]
- Got it.
- Okay. Little present, Mike. Lose it, and your a brain surgeon without a scalpel.
- Now, start loosening the screws, gently.
- When the plate's loosened, take a deep breath, and check inside without pulling the plate.
- You just blew yourself, and all of us, to Kingdom Come.
- He's learning to disconnect a bomb.
[...]