Arc motion In an articulated drive train

[cadandcookies]

As a summer project, my team has decided to build an octocanum drive as a test platform, showbot, and design exercise. We've been playing around with numerous methods for actuating the drive modules, including designs similar to 525 Swartdogs', 488's octocanum drive, and, recently, 3928's butterfly drive. I've also taken a look at 148's 2011 drive train.

However, try as I might, I can't quite figure out 3928's actuation method from looking at their design. I remember talking to one of their members at the North Star Regional and I believe they said that they used a piston with no sort of connection to the module, but that doesn't sound right to me, and I honestly can't see a piston in any of their posted photographs.

Does anyone have pictures of the design or some sort of insight into their method of actuation?