It's not exactly the same as a drybag - most drybags I've used open at the end of the bag, whereas the Rack Pack opens up longways, which gives you better access to the contents. Maybe there are drybags that open that way too, I don't know.

The Rack Pack buckles to the straps on the rollers that usually come up and over the top of the roll, in the middle of the bag, keeping the roll down. So you take the outside strap from each of the panniers and clip it instead to the rack pack. When I first tried this, it seemed rather flimsy and unconvincing, but after actually using it on a tour in 2008 I found it works well. I put a couple of bungee cords criss-crossed over the bag, along with a cargo net, and everything seemed to be fairly stable.

Here's a pic of the setup:
http://www.crazyguyonabike.com/doc/p...?pic_id=281733

Neil