It's also got best practice votes so I interpret it as "a good combination of being maintainable and efficient but not something you would expect to see in production code."

check here please: https://docs.codewars.com/getting-started/solutions/#clever

Clever doesn't necessarily mean best practices. I once saw a solution that was worded in the longest way possible (like, absurdly long) and it got clever because most people got a laugh out of it, lol.

Check Discord Fixing channel.

While this behaviour is indeed an artifact of the testing system, it is also so easily accidentally stumbled upon that I would still consider the kata to be broken. It is also very easily fixed by moving the describe and it block from preloaded to the the sample and full tests. ( Fork here which fixes it )

Not a kata issue. Closing.

Specify the language where it is broken.

is there really any reason to do it this way besides it being a cool one-liner? is it more efficient than a normal loop?

I would rather give you an advice: try to solve as many basic katas as you can (8 / 7 kyu), and try to analyze the submitted solutions to understand how they work and to reproduce it yourself. You will learn a lot doing this, and more efficiently that with a theoretical explanation. You can do it with this one: copy the code, try to do some changes and see what happens.

Could someone please explain how this line works step by step? I am confused

Nice work checking the issues, kata curation is sparce on CW.

Nothing about this qualifies as best practice :/ I hope whoever voted it so clicked Best Practices instead of Clever by mistake.

Yes, that was pretty useless.

Fixed.