Originally Posted by bergermeister

Ah, gotcha. They seem to lock very securely. No movement once engaged.

Imagine two pieces in the shape of the letter "C" secured together in parallel, but only one is able to rotate. When the rotating "C" is open, the two are in line and just make a "C" with an open end to drop onto the rack bar. To close, rotate one of the "C"s around the rack bar such that it is now opposite the fixed "C," which now forms an "O" around the bar. For orientation reference, the "C"s point downward, as would a hook in your link above. Hope this makes sense.