Ippatsu 「一発」 is a yaku completely dependent on riichi. 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. 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.
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. Thus, players may actually use the option to make tile calls to deliberately deny a player the chance for ippatsu.
- Ippatsu in Japanese Wikipedia