The Dahon Neos is specifically designed for use with small wheeled folding bikes. It mounts by bolting into a special dropout in front of the rear wheel. This enables it to protrude very little from the frame and it also allows the cable routing to be very straight. Both of these are important features which minimise damage when the bike is folded. It is also a low normal design with a very short cage - designed for ease of use on bikes with a single chainwheel.
It was developed by Dahon in association with SR SUNTOUR, and you can see some evidence of the old, crazy, SunTour, freethinking DNA in its design (see the SunTour S-1 and SunTour SX-100). I like the fact that it has a nine tooth guide pulley and a ten tooth tension pulley - eccentricity is not dead.