I'm not sure why this event has survived, but some must enjoy doing it and I suppose it's no more unusual than say the pole vault, although the latter is entertaining and the skills are to be admired- whereas this fast walking just looks silly
It's always looked like a recipe for arthritis in later years to me.
To answer the question posed though, I believe the key criterion is that a contestant must appear to be looking desperately for a toilet but unwilling to stop if they find one to be judged to be walking.
Surely no more unusual than the hop, skip, and jump. Wonder what they opted to leave out when designing that event. Headstand ? Cartwheel ? Backwards flip ? Pirouette ?