"I was drawing KeinexKeine when a Mokou who hasn't sprouted anything in particular burst in and started thrusting her hips, so it became a MokouxKeinexKeine pic. It's definitely ins..."
That last truncated sentence in particular, "It's definitely inside, huh?" (kore zettai haitteru yo ne, これ絶対入ってるよね), is a Pixiv meme applied mostly sarcastically to work-safe pictures where characters are posed such that it looks like penetration could be occurring (post #923605 has it, for instance).