Note the small, black, rectangular block just below the brake calliper. This holds a pulley wheel. The Bowden cable comes in from below, and the inner is looped around the pulley and taken back to the small chain that operates Sturmey-Archer gears (here the two have been separated). What makes the five-speed Brompton triangle unique is that it has two of these pulleys, one on each side of the frame. The one on the right (that you can see in the background) carries the Bowden cable that selects the three ratios for which Sturmey-Archer is famous. The one on the left switches the gearbox between wide range and narrow range.
