The Canon Tripod Mount Ring C (WII) is shown above mounted to Canon EF 70-300mm f/4-5.6 L IS USM Lens, which is mounted to a Canon EOS-1Ds Mark III.
You can mount this camera and the 70-300 L lens to a tripod without the tripod ring (the camera body tripod insert can be used), but the setup is very noticeably front heavy. The centrally-positioned Canon Tripod Mount Ring C (WII) provides a much better tripod balance - and results in much less movement after the tripod head is tightened.
The thumbscrew tightens/loosens the tripod ring friction as needed. Simply rotate the ring to the desired position and lock it tight. If not needed, rotate the ring to an out-of-the-way position when not in use (note that the Canon Tripod Mount Ring C (WII) may bump into certain battery grips). Alternatively, loosen the tripod ring locking knob and pull out on the thumbscrew to completely release the ring from the lens. This release is accomplished via a hinge (noticeable in the product images above). The hinged design allows the Canon Tripod Mount Ring C (WII) to be removed without the need to un-mount the lens from the camera.
