I took a quick picture of the inside of an old chubb safe i worked on just to show how the tamper mechanism works.

Older safes were prone to a destructive attack whereby the lock was hammered away from the face of the safe..

In this case a bracket is affixed to the back of the lock which holds a large ball bearing in recess.

Hooped around this Ball bearing is a wire attatched to a spring loaded deadbolt. (pictured bottom left)

When the lock is forced backwards the ball bearing is dislocated and allows the spring loaded bolt to engage, securing the safe.

This mechanism can also be triggered in old safes that have never been serviced and parts may have come loose.

A customer will usually describe the 'PING' of the ball bearing landing in the cup below it when calling a locksmith regarding their safe failure.