I prefer the piezo-electric idea... the piezo crystal will generate a change in resistance/electric potential EVERY time you tap it - regardless of whether you've taken your other finger off it already, and how clumsy you are at walking. There would be no "sweet spot", and the concept of pre and post-travel would be eliminated, since tapping it in whichever position would still generate the required change.
(yes, it's already been done... let's see how long before a link gets posted)
(yes, it's already been done... let's see how long before a link gets posted)