Of this bike, Sheldon Brown writes, "Disclaimer: This is a highly-advanced hack! Do not attempt it unless you have great confidence in your metalworking and mechanical ability. This article assumes that you are already very familiar with the inner workings of Sturmey-Archer AW 3-speed hubs. If you have any questions that you can't figure out the answer to from this article, you're not ready for this!"
Read the details and see more photos of this "bichain fixed/free bicycle drive system" at Sheldon Brown's website of wonderful things.