Testing bike provided by Favorit1970 (Ivo). Thanks!
Two-speed gearbox with a backpedal brake. On the outside, it looks much like a classic single speed hub, just a bit bigger. Its specialty is lack of cables: just backpedal a little (right before the brake engages) and the gear changes.
First gear is 1 (direct drive), second is 1.38 (overdrive). Besides the backpedal brake version, there is also a disk hub and a brakeless one. The driver accepts sprockets from 13 to 24 teeth and is surprisingly compatible with Czechoslovak Velamos components, making a swap easier. The owner mounted the hub in an Eska folder with 20" wheels, 175 mm cranks and chain transmission ratio of 46/16: one gear for the flats, the other for takeoffs and hills - just enough for a city. Having no shifting cables flapping around makes folding easier.
Within the hub, there is one epicyclic stage with fixed sun gear and three planet pinions, sprocket is fixed to the planet cage. Gear ring continues with shifting ratchet with alternating higher and lower teeth:
Backpedaling turns this ratchet backwards, making two shifting pawls jump to a next tooth. Their other ends then either lift and engage with teeth on the hub shell (overdrive), or sink and let second ratchet engage (direct drive). Shifting is totally foolproof: all you can do wrong is to not backpedal far enough and stay in the original gear, or to backpedal too far and brake in addition to shifting. There are no "neutral gears" or nonfunctional intermediate positions.
Second ratchet is driven by the planet cage via a five-toothed clutch. The angular play in it allows enough backward movement to shift and brake before its teeth engage. Forward pedaling disengages the brake, then the clutch engages and starts to drive hub shell via second ratchet (or, if overdrive is selected, first ratchet overtakes it and the second ratchet freewheels).
Brake ring is operated by five rollers pushed outward by angled teeth on the clutch body. The brake is pretty sharp in the 20" rear wheel of the folder, and feels the same after 3000 km of break-in as it did when new.
The hub is well greased, so if it works, it can run for years without maintenance. Disassembly begins with unscrewing 23 mm nut and locknut on the left, the whole internal assembly then falls out through the right side. When removing the brake ring, watch for rollers, their cage doesn't prevent them from falling out. Last parts requiring tools are two circlips on the axle, the rest can be taken apart by hands. The gears can be disassembled too, their pins are bolted on, nothing is riveted.
During assembly, beware of a trap: the little ring marked with red arrows in the pictures above and by number 22 in manufacturer's part list must be oriented with its stepped side to the right (towards gears and sprocket), flat side left - exactly opposite than is drawn in the part list. If installed backwards, it blocks shifting pawls and you can't shift to overdrive.