I wrote THIS POST ages ago now, August 25, 2007 to be exact. My time flies!
I call any button in Revit that doesn't look like "normal" buttons that we are used to, "Sneaky". The most sneaky of all is the little rascal in the Family Editor that lets you associate parameters to other parameters. This is the one.
Yeah the one with no tool tip...no hint what it might do. For those afraid to click on stuff it can be positively paralyzing. Just try telling someone like that to click and watch the hesistation. They're thinking, "Is Steve trying to make me crash Revit or what?".
I think this sneaky button is definitely a Reviteristic.