This article by Matt Blaze, “Safecracking for the Computer Scientist“, is very long, but quite informative. The basic way you open a real safe is the same way I would ”crack” my toy safe as a kid.
Combination lock tumblers (the wheels which turn) are imperfect mechanical devices, and as such, they leak information about the internal functioning of the lock. You don’t gain information by sound, as they portray in the movies, but by feel. The play, or give, in the lock provides enough information to determine its combination, since when any given tumbler is in the correct position (its notch is lined up with the metal insert pin) the safe handle will behave slightly differently than when it isn’t. Safe manufactures know this, and so make things challenging by modifying the tumblers, etc., but experts can still open such locks.
