[BigglesZX]

I too have seen this before, but I believe the point that the author was making was an observation of the hand signal's simplicity.

It's an unabiguous indicator of the physical situation he's observing, and doesn't involve any added complication of an ad-hoc signalling system (like waving for "come on" or "slow down" or "left/right", which are often somewhat ambiguous or at least require a second or two's thought).

A fleshy progress bar, if you will!