A hybrid of three-speed epicyclic gearbox with a classic derailleur cassette. It can replace a front derailleur giving a standard 3×something gearing, while eliminating chain crossing and providing some shifting when stopped. Or it can be combined with FD, extending your overall gear range. It weighs a tad more than a standard cassette hub + front derailleur + two chainrings. Besides the disc brake version, there is also a brakeless CS-RF3. Sram Dualdrive is a similar product, but it is no longer in production.
Eight or nine speed cassette fits the splined driver (they are equally wide on the outside and only differ in sprocket spacing), the manufacturer doesn't provide any tooth count limitations. You can control the gearbox by any front derailleur shifter (I've tested some Shimano); only the middle position needs precise adjustment, the two end gears have very wide tolerances. Pulling the cable shifts to slower gears, which is the opposite of front derailleurs (but the same as rear ones). The cable enters the hub via hollow right end of the axle, a plastic cover is available for protection. The gearbox behaves the same as the previously described S-RC3, it just makes different noises (which means different internal ratchets).
I use the hub on a recumbent cargobike, where the possibility of downshift for takeoff after a sudden stop is priceless, and the single chainring greatly simplified chain routing. Chain transmission is 38-tooth chainring and a 11..34 8-sprocket Megarange cassette. So far the hub works reliably and shifts faster than a derailleur. After first 2000 km, loud bang sometimes sounds when taking off at low gear and full power, as if a pawl slipped and jumped in place (probably harmless, nothing broke yet). It doesn't happen when I pedal more smoothly. After 5000 km I heard first squeal and then ran into a dead end: the thing is undisassemblable (does that word exist?)! Left cone can be unscrewed, cassette is easy to remove, but the flange behind it is surrounded by spoke ends and is unreachable by any wrench or vise. If you don't want to unlace the wheel, you have two choices: either make an offset wrench of an iron bar and several hardened pins which fit the dimples in the flange and don't touch the spokes, or give up on disassembling, unscrew the shifting chain and squirt some heavy oil through the hollow axle. I cose the latter: 2 ml of some transmission oil immediately cured the squeaking, nothing has leaked out yet and the hub works same as before.