I'm a complete idiot, but I've never heard of anyone using a flow-switch to prevent backflow in a WC setup. No one ever worries about it, because the only way you'll ever get backflow is if your pump dies (if then), and you'll have other things on your mind when that happens, I guarantee.
The only place it might come in handy is if you were to make a WC system using two pumps in parallel, so that if one pump were to fail, you could prevent backflow through it (this backflow would be caused by the 2nd pump). Maybe there's another use for it, I don't know.
Of course, the only way you could possibly build a WC system using two pumps in parallel is if you were someone with no grasp of physics and no tolerance for criticism, who is also prejudiced against New Zealanders. You don't seem like that type.
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