folder.fanatic posted this in the Tikit vs Brompton thread. It is from the BF site and is a wery good description of the system.
SRAM Dual Drive 3 speed INTERNAL HUB: This is an innovation that works particularly well with the Bike Friday's 20" wheel diameter. The system looks like this: single front chainring (no front derailleur), and in the rear wheel, an enclosed, planetary gear system that performs the same function as the front chainrings, in combination with a standard 7,8, or 9 cog cassette to give a total of 21-27 gears.
The advantages: Wider gear range than a triple chainring, robust, bullet proof casing which makes it great for touring. If you climb really steep hills or like really low gears, this is it! Unlike earlier editions of this hub, you can shift it while pedaling.
The disadvantages: generally a pound heavier than a typical chainring setup, and though it is a reliable component, it is not as easy to repair on the road in the unlikely even that it fails. However, it is designed to be a fully enclosed, relatively maintenance-free system.