James Farrar:
Zig-zag is three uses (in-out-in or out-in-out or purchase-in-out) in
quick succession, right?

M.I.G.:
What use would that be to anyone? I mean, what scam would be pulled by
that means?

Tom Anderson writes:
Alice and Bob buy a ticket and go to a station. Alice uses the ticket to
pass an entry gate, puts the ticket into an exit gate, retrieves it,
passes it over the gateline to Bob, who then uses it to pass through an
entry gate himself.

Right, got that.

It's passback, but slightly more subtle. A system which defeated passback
might not defeat zig-zag.

Why not? There are still two "in"s in rapid succession there, which
could be trapped.
--
Mark Brader | "Design an idiot-proof system, and the universe
Toronto | will spontaneously evolve a higher grade of idiot
| that is able to circumvent it."