A song by the Touhou doujin music group IOSYS, popularized by the accompanying viral Flashmusic video. The original version of this music is Reisen Udongein Inaba's theme music, "Lunatic Eyes ~ Invisible Full Moon."
The song is officially titled Stops at the affected area and immediately dissolves ~ Lunatic Udongein (Japanese : 患部で止まってすぐ溶ける ～ 狂気の優曇華院), but it's usually called "Overdrive" by English-speaking Touhou fans because it's the first word that appears in the music video.
