Underpromotion is choosing a knight, rook or bishop when a pawn reaches the last rank, instead of the usual queen, because that lesser piece does the job better.
Almost always you promote to a queen — it’s the strongest piece. But occasionally another piece is stronger for the position at hand, and the rules let you pick any of the four.
The two classic reasons are a knight that gives check with a fork (winning material a new queen couldn’t reach), and a rook or knight when a fresh queen would actually stalemate the enemy king, throwing away the win. A bishop or rook is also chosen, rarely, to avoid stalemate.
When you reach the eighth rank, pause for a second: a queen is the default, but ask whether a knight fork is available or whether a queen would leave the opponent with no legal move. If so, underpromote.
Two reasons: a knight can give check and fork pieces a queen can’t reach, and sometimes a brand-new queen leaves the enemy king with no legal move — stalemate. In those cases a knight or rook wins where a queen only draws.
Rarely in practice. The default is always a queen. Underpromotion is a tool for the specific moments — a knight fork or an avoid-stalemate situation — where the queen is genuinely worse.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.