"Soiyassaa!" was first uploaded on October 10, 2009. In Shuujin-P's video description he mentioned how "he wasn't confident on how good the song was but he wanted everyone to listen anyway".
A year after the song was uploaded, when Project Diva Arcade was looking for songs in the game, Shuujin-P uploaded the "Project Diva Arcade" version (it became shorter and the song's quality was better). It was later chosen as one of the songs to be on the game Project Diva arcade, and later featured in the game Miku Flick as well as a part of "Summer Pack". A Project Diva PV of the song was also chosen uploaded from an unknown user (the uploader's account was removed) to be chosen as the game's video.
The song tells a story of battle, to be more precise, the "spirit of a battle". Len and Rin can be heard shouting throughout the song (one of the examples is the title itself, which can be interpreted as a shout / battle cry "SOI YASSAA!"), and that is often done in Japanese festivals and such.