The English Teacher in Japan blog has the following quote on this concept of Kancho (among another discussion about large penises).
Let me introduce you to a game Japanese kids like to play called "Kancho."
Actually, it's not so much a "game" as it is kids clasping their hands together, sticking out their first fingers, and shoving them up your butt. I'm really not joking.
However, a bit of googling only finds that exact same paragraph in numerous locations and other 'I hear there's this game' type postings. Therefore, I'm left to wonder if Kancho really exists, or if it is just a funny meme. Then again, it does bear a striking resemblance to boonga-boonga which is equally disturbing and so I wouldn't be surprised if it did exist.
Note: a 'spanking' version of boonga-boonga appears to have been made available as evidenced by this flyer, though astute observers will note the holster holding what may be a convenient mechanical finger.