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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