like an idiot I left it at work yesterday. My boss is also the Engineering Manager and he likes to have unusual mechanisms brought in and shared. you never know where inspiration will come from. This ratcheting mechanism certainly qualifies.
The first stroke of the drive arm makes it move out, then next stroke makes it move in, and so on. It basically is holding the "embroidery" material first to one side of the stitch, then the other. the result is the top material is held in place on the surface by the regular stitching.