Are you familiar with the toggle bolt of a Luger P08 pistol. One end is hinged to the rear of slide, other end to bolt & then a hinge in the middle. When the bolt closes the hinge in the middle comes below a straight line connecting the rear pin & front pin. This is over center, thus pressure applied to the bolt trys to push the hinge downard, where it is restricted, rather than up where it would open. As the slide moves rearward a cam section in the frame lifts the hinge to a point above center at which time the barrel & slide are stopped & inertia keeps the bolt moving rearward as the hinge continues rise against spring tension only.
Any devise where hinge sections line up & then go a bit beyond where they are stopped is an over center devise. Ordinary loop closers as on a suitcase work by going over center. As you close it after placing the loop over the stud & push down on the opposite lever it will be under some pressure till thhr center point is reached. There it will suddenly snap closed & remain untill the lever is again pulled up. In opening it will be under tension until the center point is again reached & then will open the rest of the way with no restriction.
