Place a bid below and watch the price move on every other open tab with no reload. Then scroll to the collision lab and run two bids into each other on purpose — it shows exactly what stops both of them from winning.
This is the one question every auction has to answer honestly: if two people bid the exact same amount at the exact same instant, can both be told "you won"? Below, two bidders fire the same bid at the same moment against a disposable practice lot — run it once the way it's easy to build it wrong, then run it the way this stand actually works.
Not a list of technologies — the mechanism that makes the lot above trustworthy.
Every bid is a single, uninterruptible step: read the price, compare, write. Nothing else can touch the lot in the middle of that step, so two bids can never both come out as the winner.
Every visitor sees the same countdown because it isn't running in their browser — it's read from the server on every update. A bid in the closing seconds pushes the deadline back instead of sneaking in under it.
Price, leading bidder and countdown are pushed to every open tab the moment something changes — the page you're reading right now is already listening.
A bid that loses the collision isn't silently dropped — it comes back with the price it lost against, so "you were outbid" is never a mystery.