Complex sentence: When some other piece is chosen it is known as "underpromotion", and the piece selected is most often a knight, used to execute a checkmate or a fork giving the player a net increase in material compared to promoting to a queen.
Split into simpler sentences: When some other piece is chosen it is known as "underpromotion". In underpromotion, the piece selected is often a knight, used to execute a checkmate or a fork giving the player a net increase in material compared to promoting to a queen.