When you play a race or battle in multiplayer, the points you obtain after the match ends will be added to the gauge.
You can fire the pipe once when the gauge fills up based on the points you obtain.
You can save up multiple pipe fires.
When the tour switches, the number of fires you have saved will not change, but any progress on your current gauge will be reset.
If you are subscribed to the Mario Kart Tour Gold Pass, the gauge will fill twice as fast.