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yeah... but I mean that's why they are always falling on the floor

It's a variety thing in Japan.
When someone says something you wait for the "point" of the joke to come, but when it booms (すべる: slips=fails to be funny) it makes you not laugh but instead just fall over cause it's so bad.
Can't explain it any differently, maybe someone else can. I know it's difficult with the language barrier, but watching more things (I know there are quite a lot of subbed variety out there) will make it easier to understand the comedy culture of Japan, if nothing else.

The slipping/falling is a comic exaggeration of how, when you pay close attention to someone or something, you tend to lean in and ignore whatever else you were doing. And the longer you wait for the point, the more you lean in, so that when the point comes but is not what you anticipated, you are dazed for a moment and must recompose yourself.

The old vaudevillians did it; just think of the reactions of Lou Costello and Jerry Lewis. The Japanese have simply taken the same gag to its logical-but-ridiculous conclusion, i.e., when a joke is so bad it literally knocks you off your feet.

It helps that the Japanese are more comfortable with sitting and lying on the floor. It's part of their normal living space, as opposed to some sort of forbidden zone like it is in much of the rest of the world. (Although that seems to be changing as they become more westernized.)

See also…

Celedam wrote:Literally, it means to slip or slide. In the context of telling a joke, it means to bomb or fall flat. Erina and Ayumi are the suberi combi -- or simply "Suberiz" -- because they're bad at telling jokes.

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Captured these two from J-POP Ranking.

Captured these from the credits of her recent solo DVD.

^ I'd have to watch other performance, but I'd say that was on purpose to help get her in position. With the way the bit before goes she ends up on the wrong foot to be able to do it like everyone else. Could be wrong though

And re: forever ago, but I... uh... usually fall down laughing? Like when I'm with friends I will laugh and drop to the ground and then roll around depending on how funny things are. I can remember doing that as far back a middle school Hahaa.

hmm... Well, if you look at Meimi, she clearly is able to hoppity farther than Kana has to. :3 and also Rina.
Also the little hand jerks she does. She's so cute.