The surface going on the carpet is file cards - the short straight prickly metal brush on the tool you use to clean a file. The metal teeth of the brush dug into the carpet, basically couldn't be moved while in, and had to be picked straight up. It used pairs of the file cards to walk/march down the field. This also means the robot couldn't really ever turn itself, which was one of the only weaknesses of the design. This file card drive is believed to be the reason for the "no metal contacting the carpet" rule.
Source: I know the guy who drove it
Edit: If you mean what kind of linkages or how they produced the marching motion, I'm not sure, but I can probably get him on here to answer if noone else knows.