The Chrosziel DSLR Anti-Twist Plate is a p-shaped replacement part that goes on the 401-421-S lightweight camera support system, which is a system specific to the Canon 5D and 7D and the Nikon D90. The plate mounts behind the camera, onto the top plate, and keeps the camera from twisting one way or the other.
This plate is called a replacement part because the 401-421-S already comes with one.