[Kevin Sevcik]

Alright, a more long-winded explaination appears to be in order to answer everyone's questions. Not that I built the thing, but here's my take on how it's working.

1. Both motors are tied to the bottom large gear. Probably using small gears so they're at the same ratio.

2. The bottom big gear meshes with the medium gear above it. This gear is on the same axle as the gear to its left, so they both turn.

3. The medium gear meshes with the small gear above it, while the large gear next to it doesn't mesh with an extra gear.

4. The shift happens by moving the top two medium gears from left to right. In the right position, it's effectively just the large bottom gear meshing straight with the medium gear at the top for a large-medium step-up. In the left position, the top medium gear meshes with the large gear just below it. So there's a large-medium-large-medium step-up here.

Caveats: The tranny as shown does not actually shift on the fly, as there's nothing to move that top axle left to right. Probably a servo could be rigged up to do it. Not that shifting on the fly would be particularly wise. The extra small gear in the train when the right side is engaged means that this gear is reverse of the left gear. A nasty sort of thing to do to gear and motors and such, though these are small and weak and all.

Suggestions: Perhaps the extra small gear can be dropped, and the left medium gear be replaced with a small gear to keep the center distances compatible. I think this would work and get rid of the reversing. Not to mention giving a bigger shift.