Ippatsu

Ippatsu is a one-han bonus yaku in Japanese mahjong awarded for winning within one uninterrupted go-around immediately after declaring riichi. Concretely, you score ippatsu if you win, by either self-draw or ron, before your next discard, and crucially before any opponent makes a call (pon, chi, or kan) that would interrupt the natural turn order. It can only ever accompany a riichi declaration; it is impossible without riichi and impossible in non-Japanese variants.

The single most important rule about ippatsu is that any call breaks it. If, after you declare riichi, an opponent pons or chis someone's discard, or declares a kan, the ippatsu window is cancelled even if you go on to win shortly after. This makes ippatsu fragile, and skilled defenders will sometimes make a call specifically to deny a riichi player their ippatsu and the extra han it represents. A closed kan by anyone likewise interrupts it.

Because it stacks with riichi and frequently with menzen tsumo and dora, ippatsu often pushes a hand up a scoring tier, for instance turning a 3-han hand into a 4-han hand worth substantially more. It is purely a bonus of timing rather than hand shape, so it cannot be planned, only hoped for.

For example, you declare riichi on your discard while waiting on 4p or 7p. On your very next draw, with no opponent having called any tile in between, you draw 7p and declare tsumo: you score riichi, ippatsu, menzen tsumo, plus any dora. Had the player to your left ponned a tile during that go-around, the ippatsu would vanish even though your winning draw was identical.