How does the BBB (or the other external, clamp-style) extender work? For instance, does it come with its own star nut, so that you adjust the new stem with the vertical bolt screwing into a star nut that is inside the extender? Or does it come with a really long bolt that screws into the steerer tube's star nut?

If the extender comes with its own star nut, then that means that the two short bolts at the base of the extender (the clamp) are the only thing that attaches the extender to the steerer tube. If so, how would you make sure you have enough pressure down, and how would you fine tune that pressure (as the vertical bolt would do) so that you have no extra play, and no excessive adjustment either?