They're a lauan plywood frame under carved insulation foam, knees hinged with cord, and secured to a belt over a black apron the actor wears over his biological legs. Two puppeteers operate the fake legs with rods, with the right lighting giving the effect he is floating in the sky. It's pretty simple, practical theatrical fun. Here's the test maquette:
And the final legs in progress. I love this electric turkey carver!
The tricky part was making hinged rods so the legs could be operated from above or below. These are PVC pipes wired into a pivoting joint inside the calves.