First attach to lower end of vertical rail then attach to lower rail using the standard vertical rail mounting knob. After you shoot your sequence of shots and ready to shoot the nadir (down shot) simply loosen the Nadir Adapter using locking lever and rotate the vertical rail 180 degrees then tighten. It is good to use a reference point on the ground as you will need to move the tripod assembly slightly so the lens is pointed directly over the original point of rotation. The Nadir Adapter is a must have for anyone shooting an unobstructed down shot, especially important if shooting HDR.