I had a go with the Offcuts walking foot at a show a few months back. I love the “Heath-Robinson ” effect, and, like the original Heath-Robinson devices, it is surprisingly effective. On a theoretical level the foot “pace” length is fixed while the underneath feed dog driven system varies with the chosen stitch length. I have visions of the thing going round in circles or something. However a bit of thought shows that the usual foot has a “zero” pace and it works well enough.
Foot pedals do have a hard life and are hard to fix. As Offcuts says, best left to those who understand them.