Gyroscopic Investing

Basically, if the vault senses it's being tampered with, a spring-loaded mechanism fires like a mousetrap, snapping this metal block into place, that physically makes it so the door can't open, even if you have the combination.

Dave McOmie:
And when it fires, it goes, bang! It shoots over, and you cannot turn the handle. And I was really nervous about the mousetrap relocker in this particular door.

Several Mosler vaults installed in Hiroshima's Mitsui Bank building prior to WWII survived the nuclear attack, a fact the company widely publicized in its marketing.[3][4]