... he times his shot to capture people at the point in their stride when they look as if they're standing still...
I don't know the Harry Gruyaert (I should probably be embarrassed), but I would guess the technique he's using is taking a burst of photos, not timing. He can then can publish the best shot.
In my experience, photographers use that technique to greatly improve their chances of capturing the exact right shot when motion is involved. It also provides a way to minimize camera shake for slower exposures. Long ago, burst photography required a motor-drive and it used up 36 shot film like a machine-gun firing. With digital, burst can be used freely. Even on a cellphone camera. Deleting images is preferable to not getting the shot. You can't sell a shot you didn't get, and you must maximize every minute shooting to make a living at street and candid photography.