Why an air gap is the only truly fail-safe cross-connection control — and why mechanical devices remain the practical choice everywhere an air gap won't physically fit.
Once you understand that backflow can happen two ways — a supply-side vacuum (backsiphonage) or a downstream overpressure (backpressure) — the next question is how to actually stop either one from reaching the potable supply. There are really only two families of answer. One removes the physical connection between the supply and the hazard entirely, so there's nothing for either mechanism to act on. The other leaves the piping connected but inserts a mechanical device engineered to physically block reverse flow when it happens. Both are legitimate, code-recognized approaches — but they are not equivalent in reliability, and knowing why is central to cross-connection control.