Promotion
A pawn that reaches the last rank must immediately become a queen, rook, bishop or knight of its own colour. The choice is free and unlimited — you may promote to a piece even if one of the same kind is already on the board. Underpromotion to a knight is a genuine attacking tool when a queen would give stalemate or miss a fork.
Start
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1.
The pawn leaves the board and a queen appears on a8. Writing a8=N instead would underpromote to a knight — the attacking tool from the definition.