[Tasked]

The article below might help. Note that the drumsolo before the performance is a Taiko-drum solo. Those are taikodrums they use, and that's why I think this article has something to do with why they shout that word.

"Kuchi shōga (口唱歌), also known as 'kuchi showa' and 'kuchi shoka', is a Japanese system for 'pronouncing' the sounds of drums, especially Japanese drums (taiko).

Although specific usage of kuchi shoga varies, certain words are relatively universal. For example, a deep, sustained sound from the center of the drum might be rendered as "don". "Do" sometimes represents a short beat that is not allowed to resonate, and "doko" often represents two beats in rapid succession. Two quick beats may also be pronounced "dogo" or "doro," depending on the song or group. "Tsu" represents a lightly struck note, and a sharp tap on the side would be "ka." Two quick tsu's or ka's would be pronounced "tsuku" or "kara."

Some taiko groups use "do," "tsu," and "ka" to represent a hit with the right hand, and "ko," "ku," and "ra" to represent a left-hand hit. This allows the "sticking" of the song to be conveyed by the kuchi shoga.

Rests or pauses are often sung, using words like "su," "hup," or "sore" (a two beat rest, pronounced "SOH-RAY"). If the rest is not sung, the space is often filled with unscripted kiai's. Explicitly assigning words to represent the periods of silence in a song is likely linked to the Japanese concept of Ma, where the space between notes is as important as the notes themselves in a performance.

Example: don sup doko, don sup doko, doko sup don, in 4/4 time is three measures of beats: 1 . 3 4, 1 . 3 4, 1 2 . 4

The system is used for recording, teaching, and memorization of drum rhythms. It is also used for 'subtitles' in the Taiko no tatsujin drumming games."

source: wikipedia