Otherwise known as DDong Chim, this the act of anal finger thrusting. Performed on unsuspecting, unguarded anuses, it is extremely popular with the Korean kids.
If you’re interested in doing it yourself, it’s really quite simple. All you need to do is clasp your hands together, fingers interlaced, with your pointers sticking out together, like when you shot pretend laser guns as a child. When you see someone bending over or otherwise with their backs to you unawares just dive right in. You’ll be surprised at the success rate of such attempts. You see, at any given moment, not a lot of people are expecting to suddenly feel fingers in their butt.
(Thanks to metamorphaillic.wordpress.com for this insightful information and picture)