What kind of move is this

Can someone explain how this move works 

Thanks cap

That moment when you thought it was checkmate...

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until the "weird move" comes.

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En passant is normally the last special move learnt by players. The position above fits the criteria of en passant:

- The Black pawn is on the fourth rank (fifth from Black's point of view) on b4

- A White pawn from the adjacent file (c-file) along the second rank (i.e. c2) moves two steps forward

- The capture is made immediately on the first opportunity

- The new position of the Black pawn would be on the third rank (and the final position appears as though White has only moved his pawn one square forward and Black captures this pawn)

I can confirm that the move Black played is called "En Passant".