“What Am I Doing Series” (何やってんだ俺シリーズ, Nani yattenda Ore Shirīzu) refers to an animation trend of having breasts bounce to music, usually Spelunker, that stems from a Touhou Project fan animation that was uploaded to Niconico in July 2011.

On July 29, 2011, a user named “Roah” had uploaded a brief animation of Utsuho Reiuji jumping to the main theme of Spelunker, causing her breasts to bounce with every jump.¹⁾²⁾ The video had the simple title of “Unyu” (うにゅ), but had the description of “What am I… doing…” (何やってんだ・・・俺・・・) that ended up naming the trend of similar animations that emerged over the following days.³⁾⁴⁾

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• There is a specific tag for Touhou Project derivatives at “Touhou Nyūshindō(Breast Vibrations)” (東方乳振動), but it appears to be underutilized, people aren't aware of it, or it keeps getting removed.
• Due to the rather unconventional title and tagging schemes, most of the re-uploads to YouTube were retitled as “Caution!! X Bounce!!” instead, with “X” being replaced with the character.

• Unyu on Niconico Douga - The original upload.
  • What Am I Doing Series on Niconico Douga - The tag's search results.
• What Am I Doing Series on Niconico Encyclopedia