Underpromotion
Underpromotion chooses a rook, bishop, or knight instead of a queen because the queen would fail or stalemate.
What to look for
Check whether a knight promotion gives check, whether a rook/bishop avoids stalemate, or whether queen promotion allows defense.
Recognize the mechanism
Compare the legal promotion choices by their actual attacks. A knight may supply a unique check or fork; a rook or bishop may avoid a stalemate caused by a queen. Calculate the continuation for the chosen piece.
Common practical mistake
An unusual promotion is not an end in itself. Show why the queen fails or why the other piece produces the required result.
Training goal
Train accurate promotion choices instead of automatic queening.