1) I have loved doing it since the age of nine.
2) When people do it to me, as well, I can't help but laugh.
3) This is a prime example of something that can be defined in a single word/phrase in Japanese, but which I think requires a subject-verb-predicate sentence in English. There are quite a lot of these, and I often find them somehow charming.
4) As I just learned (from searching for visual examples), there seems to be a sort of trend in making domino-like videos of this phenomenon.
So, first of all, I was taught this phrase some years ago. One of my students was standing right in front of me, leftward leaning so perfectly as if to be saying, "Hit the back of my knee! Hit the back of my knee!" I (gently) used my knee to nudge the student's. Someone behind me burst out, 「ひざかっくんだ！」。 ひざ, of course, is the knee. I guess かっくん is what we do to the back of the knee. As a verb, we would say ひざかっくんする, so it's a suru verb. My students asked of me the English equivalent, and the best I could come up with was "hit 'em in the back of the knee."
When I Googled it, the second result was an app on iTunes; someone actually made an app out of it:
In case the description in the screen capture is too small to make out, it's written thus:
Description
It's a silly game.But very famous game in Japanese kids.
We have made this game as an app!
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This one is just, you know, how it's usually done. One person sneaks up on his/her friend. If the person about to be tapped is standing properly, balance evenly distributed, then go for the backs of both knees.
But here it's taken it to another level. I feel that the groupness of the culture shines through clear and bright.
This is how they do it in the animal kingdom.
My beagle actually did it to me once or twice--just ran up and threw her body into the backs of my knees. I wish I had video of that.
And, of course, it needed to be made into a song. The world would be a darker place otherwise.
