THere's a pic of Kanako doing that and even of Miku (though with panties, not pants) but for the life of me I can't find and link it for reference.
I saw this on 'Gaki no Tsukai' once. Some comedian was thrusting his arm up in the crotch region of some very stretchy workout pants and shouting each time he did it. It was hilarious.
In summation, I think this is just some japanese meme we're not savvy on in the west, but it's adorable and Okuu-licious all the same, so I accept :D
