How it works: the room is told the mark — “STOP ON 10.6” — and then one clock counts up on the big screen and fades to nothing before it gets there. Everybody plays at once. Carry the count in your head and tap STOP on the number. Closest wins the round; land inside a twentieth of a second and it's a DEAD STOP.
The liar clock: once the real clock is gone, numbers start blinking back on screen. They climb like a clock and they are almost always wrong — up to two seconds either side of the truth, and honest about one time in twenty. They are there to make you doubt a count that was fine. Every screen in the room shows the same lie, so nobody can cross-check.
Why it's fair over wifi: your phone times you on its own clock and only sends the number it landed on, so a slow connection costs you nothing. Phones quietly sync to the big screen between rounds — the ⏱ sync tag on your controller shows how tight yours is.