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Piscines are intensive multi-days coding sessions that are predominant in these schools (selection for entry is done with those sessions as well).

This is to encourage the "get shit done" mantra of these schools.

It equates to (more or less) the method of teaching to swim by throwing them in a pool: swim or drown. In 42's case, it's code something or fail your class, but without any (or much) guidance, mostly in self-taught way.

I think it refers to the culture of the first selection phase which is called 'la piscine'