To add just a bit to Rabbits description, this reversal of action is quite often used as a lock. Examples of this are the toggle actions of the Henry, Win 66 & 73 rifles; the Luger pistol etc. On all of these actions once the hinge of the toggle has passed center & reverse force applied the lever is stopped from further movement thus locking the bolt against the breech of the bbl to resist bolt movement on firing. In the case of the Henry/Winchester the gun is then opened by simply moving the lever downward, while in the Luger semi-auto action the bbl & bolt recoil locked together until the hinge is forced back up above center by cam rails, the bbl then stops & the bolt continues rearward ejecting the fired case & picking up a new round.
The term "Overcenter" really has nothing to do with the position of the opening lever on a break open gun. The lever goes past center or left of center but there is no over center camming involved.
There are many clamping devices used in industry for rapid parts change in machining which use the over center principal.
