Spring clamp slips over zoom or focus ring on a lens as shown in the above photo. This clamp engages with the two ribs of the gear drive sleeve shown in the center photo. The gear drive sleeve is inside of the port and is driven by top left knob on every slr housing. Both zoom and focus can be obtained by using a focus port that also has an internal focus sleeve.

The above photo shows complete assembly of the lens, with spring clamp attached, positioned in the port. Notice the ribs of the gear drive sleeve engaging the clamp. This view is from the back of the lens where it would be attached to the camera.