Toggling over center refers to the way the press lever linkage works. On presses with a compound lever linkage there are three pivot points: one at the bottom of the ram, one in the frame, and one on the toggle link. As you swing the lever through is stroke, there is a point at which all three of these pivot points will line up, and this would be "on center". On some presses the lever linkage is free to continue on past this point, and hence the term "toggling over center". All Lee presses have built-in stop lugs to prevent the press from toggling over center.
