"Would you mind expanding on the "moving belay" comment?"
Pictures should cover it
Photo courtey of http://colinhaley.blogspot.com/no rope and a moving belay
"is it meant to convey the idea that if you fall and the tools are stacked vertically the stretchy umbicals are ok for a minor fall?"
None of the commercial umbilicals are INTENDED to hold a fall. But all of them (3 different brands) have to date held more falls than I can count off hand.
Umbilicals can do many things....they keep your tools attached to your body, hold a slip or a leader fall on a short leash, back up belay anchors or actually are the belay anchor until you can tie directly to your axe/axes if it is required or better yet a set of screws. I don't normally weight my umbilicals as anchors or climbing but have done just that while taking out screws on a TR while seconding. And I have put full body weight on them with my axes sunk and slack on the main belay anchor. The key to how you use your own umbilical system is to know their strength/test numbers and use them accordingly. Hopefully that helps.