Futsal is rather like soccer played on a smaller scale with less people, which alone drastically changes it. (Kazamatsuri Shou notes that it's actually more like basketball.)
Futsal first shows up in Whistle! at the beginning of Vol 10. BROTHERHOOD when some members of Sakura Josui goes to scope out Hiba. They find the soccer field empty, but Shou spotted the crowded futsal courts on the way in. They learn the rules in a game (which they lose) against Hiba's defenders, and later Matsushita Souju urges Shou back to the courts and he begins to play there regularly.