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Actually, it's open bolt (the difference confused me for a while) that's prone to cook-offs in an overheat. Closed (which requires the bolt move forward, and enclose the chamber completely) can also cook, but you'd be spitting lots of lead out of the barrel to be sure.
Closed bolt is a little bit complex.
___| bolt in forward position
_| bolt travelling back
|=== bolt all the way back, fresh cartridge is fed
_|== travelling back
Open bolt is far simpler:
|=== Cartridge ready
*EJECT*
|=== Cartridge ready
The bolt never actually moves. The mechanism is usually spurred elsewhere, but the rate of fire is more easily made higher due to the fact that the mechanisms are prepared in advance. It's rather dangerous and prone to safety hazards.
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