Questions show up on the big screen, everyone answers from their phone, and the leaderboard updates in real time.
Two teams, two QR codes: everyone answers on their phone, the screen counts each team's points and shows the duel.
When signing up by QR code, everyone answers the poll on their phone; the screen shows the results.
Players sign up by QR code and appear on screen, then answer, on their phones, a quiz in sync with the screen; the screen asks the questions and shows the leaderboard.