This is a type of lock that has a safety mechanism to prevent it being accidentally opened. Two actions need to be performed in order to open it, the first action trips (releases) the device blocking the locking mechanism, so that it can be unlocked.

https://www.google.com.ar/patents/US3934282
"Safety trip lock for the drop side of a crib comprising a trip wire having an intermediate offset actuator portion, said wire extending to the ends of the crib and terminating in offset portions similar to cranks, together with a torsion spring maintaining said trip wire in a predetermined position, and latches at the end of the crib, i.e., on the corner posts, for receiving said offset ends in recesses therein, such that the trip cannot be operated unless the entire drop side is raised prior to moving the trip wire actuator."


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Note added at 8 hrs (2016-01-06 09:36:22 GMT)
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https://www.google.com.au/patents/US6209770The safety trip assembly is biased toward the extended position and is moved toward ... Safety trip assembly and trip lock mechanism for a fastener driving tool

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Note added at 13 hrs (2016-01-06 14:00:32 GMT)
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It occurred to me that fitting a trip lock to open a safe door from the inside would allow it to be opened by a member of staff who knew how to use it, while trapping any careless bank robbers.