It looks like the seat moves back and forth on that arm.
The arm may be spring loaded or flexible to act as a shock.
The question in my mind is the hydraulic thing. If that works it could be curtains for chain drives on everything except racing bikes. In theory you could store energy both from braking and cranking. This could really change alot.
Gears are a bit of a problem. All I can figure out is you could have a system with mutiple pistons. In other words when you turn the crank you would able to chose how many pistons you want to send fluid to. I suspect the first version is a one speed.
BTW, I do not see brakes on that thing so it may actually brake regeneratively through the drive system.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.