The concept of the game is really well done, some things could be explained a tiny bit better and I think there is a bug somehow causing a pawn to become a king or something idk, but other than that it's good :)
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Well, the point about explaining things is definitely right ;P
When you capture a piece, you swap places with it and take control of it. This might be what you're talking about.
There's also (spoilers!) a mechanic based on a lesser-known rule in actual chess where a pawn will change if it makes it to the end of the board.