The IVB Power Switch Star Diagonal S2 was designed to allow your SCT to focus the Binotron 27 in reduction (0.66X), normal (1.15X) and 2X. These instant powers allow on pair of eyepieces to operate in three powers for the entire observing night.
The IVB (interlocking Visual Back) is especially useful for SCTs on Wedge and Equitorial mounts because the interlocking components prevent accidental rotation when the scope has slewed to a new position. The IVB thread-on section attaches to all SCTs and the IVB Diagonal has a silver "sprocket" that intermeshes with the visual back's black sprocket. The meet and lock prevemting rotation. No heavy torque is required to secure or loosen this system!