I now understand how they work from a google search of other forums since posting this question but I still have other questions.
The concept is that the case neck is tightly fitted between the de-primer mandrel and the inside of the sizing die and the case growth is stopped by a step on the mandrel. So you start with a trimmed case (0.010 or 0.020 shorter than max??? and must it be only be once fired ???) and it will never grow much more than the initial first firing after the first use of the die. It should never require trimming again in the life of the brass.
One guy there did a test on ten Lake City 308 Win. cases. He got more than 12 reloadings and the brass was still good. At 15 he concluded the test because he was starting to see problems of case separations in some of the cases. But they went 12 reloadings without a measureable problem. The case length growth stopped after three reloadings but never required trimming. The case thickened in the necks after the 12 reloading but were still reloadable. He normally got only three to five reloadings out of these same cases using regular dies and shooting them in the same gun. There was no accuracy degradation either.
His testing was very scientific and controlled and worth the read:
The fourth response.
This has prompted me to try the X-dies and I can only hope the directions packed with the dies answers all my other questions.