Two teams share one device. Each team secretly numbers the same twenty plays in its own order — so a play's number means something different to each side. On offense you call a play by its number; on defense you call a number too, guessing which play it's closest to. The closer your guess lands to the real play, the less ground the offense gains.
Standard downs, punts, field goals and kickoffs apply. First team to the score limit wins.
Whoever is playing that team should be the only one looking when it's their turn to call a play.