What does the giant screw looking thing in the middle do?
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I'm sure those who already have one in use can speak of its effectiveness and practicality. But from what I understand from reading about these crossings -- (I plan to buy two of them for my new Std. gauge layout) -- the position of the center screw determines whether the "diamond rails" of the two routes are electrically isolated or not.
For example... If you're simply using the crossing in a simple figure 8, you may want both tracks on the same circuit. However, if two physically separate routes/loops cross, then you may want the diamond rails isolated to each track. At least that's my take on it, without actually owning them yet. 
FWIW, I've heard some users indicate this little "enhancement" is more trouble than its worth -- especially if you're NOT looking to have the two tracks isolated. And it's best to hardwire the crossing to avoid issues down the road.
Hope that helps.
David
David,
I also came to the same conclusion. Not to long ago I bought one to use. Ending up somewhat confused, I decided to go with a NOS late prewar I had on hand. I donated mine to a friend. I might ad, at my age I am easily confused.
