GN 810.4 vertical acting toggle clamps have a safety function whereby, during closing, a spring loaded safety hook arm ensures a secure locking connection. It prevents the inadvertent unlocking of the clamp due to vibrations or an accidental attempt of loosening / opening of the clamp. To open and release the toggle clamp from the locked closed position, press the lever arm and safety hook lever arm together (one hand operation). The toggle clamp is then in the open position which can also be locked temporarily in that position via the hook of the safety hook lever arm to avoid inadvertent closing of the clamp. They work according to the toggle principle: lever and clamping bar move in the same direction. In the clamped position the operating lever is in its vertical position.