Explanation

Protecting a square is insufficient, if the protector can be captured.

Examples

White has a passed pawn on d7, which can promote with d8=Q.
The promotion square is protected by Black's rook on f8.
If White promotes the pawn immediately, Black can just capture the new queen, and White has achieved nothing.
However, White can win with Rxf8+, capturing the defender of the promotion square.
After the forced reply Kxf8, White can now promote with d8=Q+, and has a winning endgame of queen versus rook.

Sometimes, capturing the defender of the promotion square is worth a sacrifice, as in the following example:

Black is a knight down in the endgame, but has a dangerous passed pawn on d2.
The knight on e3 is protecting the promotion square.
Black can sacrifice the exchange to capture the defender with Rxe3!
White has no way to stop the passed pawn, so has to recapture. Black can then promote and keep the new queen, with a winning endgame.

Sometimes capturing the defender can include other motifs:

Black is the exchange down in the endgame, but has a dangerous passed pawn on e2.
Black controls the promotion square with the queen on c1, but White is protecting the square twice, with the rook on g1, and the king on f2.
Therefore the immediate e1=Q could be answered with Rxe1, and Black would have just lost the passed pawn.
However, Black can win by capturing the defender with Qxg1+!
After the forced recapture Kxg1, White's king is deflected away from protecting the promotion square, which is now unprotected.
Black can now promote with e1=Q+, and has a winning endgame.

Visualization

When you are threatening a promotion, but the promotion square is protected, always think about ways to remove the defence.

Related combinations

There are various ways to remove the defence. In addition to capturing the defender, other ways to remove the defence include deflection, and attacking the blocker.