Though it seems really weird to us, you should compare it to giving someone a wedgie, or a solid poke in the ribs. Its something cartoon characters do on Saturday morning cartoons.
Here is a hilarious video about the practice in Japan. I laughed so hard I literally made myself sick.
If you are interested in kancho/ddong-chim and Flower Sensei's tutorial did not fully answer all your questions, here is a tutorial from Kancho University on the correct technique.