Proprietary two-stage screw design. Clamping screw features two different thread pitches. The coarse 16tpi is used when no tension is on clamp, allowing rapid movement of the clamp vs others on the market. With high speed comes low torque though, so to counteract this as soon as the clamp sees tension it will switch to the 32tpi threads in the knob to give you extra torque when not using on an ARCALOCK rail. 1/4 turn releases the clamp from the detents on the ARCALOCK rail for movement, One turn fully releases clamp to be removed from rail