I'm not 100% sure how this works, but I'm guessing you order it and it automatically downloads when available. (iTunes periodically checks for music waiting to be downloaded on your iTMS Account) Either that or it automatically adds it to your Shopping Cart. Pretty cool idea.
What else is new? They're charging $18 bucks for that album. What a rip off! I think I'll grab it “elsewhere”. (if it was $10 I'd buy it, but come on!!)
UPDATE: Thanks to John for this tip. What sucks about the pre-orders is you can't listen to the Previews yet… therefore rendering this feature completely useless unless you're a diehard fan of the band. It also doesn't show the times for the songs… it displays ‘0:00’ instead. Oh well! Pre-ordering's really only for things that you're afraid of selling-out — like for example, Video Games from stores with limited stock. Since the iTMS basically has unlimited stock, it's just a silly feature.