Kiai time
For regulations surrounding kiai time, see: Ranking criteria
Fountains at the start of a kiai section of an osu! beatmap
Kiai time in osu!taiko: ecstatic pippidon and violet-tinted conveyor belt
Kiai time, or kiai, is a set of distinctive visual effects emphasising a part of a beatmap, which is inspired1 by Go-Go Time from the Taiko no Tatsujin series. Kiai sections are indicated by spraying star fountains, stars falling from under the cursor, and hit objects flashing to the beat with the frequency of BPM. Similar effects, such as frequent sideways flashes and star fountains, are also visible in the main menu.
While kiai time doesn't affect gameplay mechanics in osu!, osu!catch, or osu!mania, it provides a 20% score gain in osu!taiko.
Beatmapping
Kiai time indicator in the beatmap editor. The kiai section has orange colour on the bottom timeline
Kiai time is most commonly used for the strongest part of a song, which is usually the chorus; such sections are typically more challenging than the rest of the beatmap. Kiai time may be enabled for certain timing sections by a mapper from the Style tab of the Timing and Control Points panel, and cannot be deactivated by the player.