Learn in the textbookPhysical Security Measures
DefinitionAnti-passback records the status of each ID as "inside" or "outside" and refuses any entry or exit that does not match that status.
ExplanationIt relies on recording whether each ID is inside, so it is built into an entry/exit management system. If someone passes the card they just used to the person behind them, that ID is already recorded as inside, so the second person cannot get in. Anyone who entered without using a card is refused on the way out and gets caught.
AnalogyIt is like automatic ticket gates at a train station: a transit card with no entry record is stopped at the exit, and a card that is already inside cannot be used to enter again.
Memory hookPassback is the trick of handing a used card back to the next person, and the "anti" in the name forbids it.