Though I've had a few requests to address this topic, no one will actually believe the truth of the matter.
The first time it happened I was mildly offended but laughed it off as an accident. The second and third times it happened, I was appalled. I've been goosed, I've been 'good gamed', but I prefer both of those to being 'kancho'ed!
Kancho (カンチョー, kanchō?) is a prank often played in Japan and South Korea by young school-aged children; it is performed by clasping the hands together so the index fingers are pointing out and attempting to insert them into someone's anal region when the victim is not looking. It is similar in spirit to the wedgie or a goosing. (Taken from Wikpedia)
This was made popular by the arcade game Boong-Ga Boong-Ga which would never work in the west. I don't quite see the appeal. But to each his own. Just don't kancho me!!