Ippatsu 「一発」 is a yaku completely dependent on riichi. By definition, ippatsu requires a riichi declaration to be in effect, for an additional 1 han. Therefore, ippatsu cannot function as a stand-alone yaku.
It is awarded if the player receives a winning tile within an uninterrupted set of opponent tile draws after the riichi declaration. The earliest possible chance to win with ippatsu comes from the shimocha's discard (player to the right). The latest possible chance to win with ippatsu is with the player's next drawn tile after the riichi declaration.
Tile calls may immediately end the chance for ippatsu any time before the riichi declarer draws the next tile. This includes any calls for kan, except for shouminkan. Once again, in order to award ippatsu, the cycle between a player's riichi discard and next turn to discard must be uninterrupted. Even if the hand wins before the next tile draw, ippatsu does not count of a tile call was made previously. Thus, players may actually use the option to make tile calls to deliberately deny a player that chance for ippatsu.
- Kan is called prior to the riichi call; and the hand wins on the next draw without interruption. Therefore, ippatsu is valid.
- This hand wins on a discard before the next draw.
In this example, the hand wins from an opponent discard before the next tile draw. However, two tile calls were made prior to the win. So, ippatsu is invalid here.
Ippatsu is unique among the yaku, for it requires riichi to even be applicable. Because of this, it is impossible for a hand to with with ippatsu alone. At the very minimum, a hand could be riichi with ippatsu. Unlike riichi, ippatsu is not compatible with every other yaku. Two cannot mix with ippatsu: rinshan kaihou and houtei raoyui. Regarding rinshan kaihou, the required call for kan to invoke rinshan already cancels out ippatsu. As for houtei, a riichi declaration must be made with at least 4 tiles left in the live wall. That leaves the very last possible tile draw (and discard) belonging to the riichi declarer.
Ippatsu can combine with chankan, as the kan call is not considered complete if someone wins off it.
The last possible chance for riichi comes, when there are eighteen tiles left in the walls. This counts the dead wall after a tile draw. So, that leaves four tiles left for regular player draws; and barring any tile calling, every player would then have only one tile draw left. For haitei then, the fourth remaining and last tile tile draw goes to the riichi caller. Naturally, tile calls may occur beforehand to deny both the ippatsu and haitei chance.
- Ippatsu in Japanese Wikipedia