Explanation

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

Examples

White could win with the knight fork Nxc7+, giving check and attacking the rook on a8 at the same time.
However the immediate Nxc7 does not work, because the knight on a6 protects c7.
White can win by capturing the defender with Bxa6.
If Black recaptures with bxa6, White can win with Nxc7+, because c7 is now unprotected.
If Black does not recapture, White will have won the knight a6.

Black could win with the queen fork Qc5+, giving check and attacking the unprotected bishop on b5 at the same time.
However the immediate Qc5+ does not work, because of Nxc5.
Black can win by capturing the defender with Bxb3.
If White recaptures with axb3, Black can win with Qc5+, because c5 is now unprotected.
If White does not recapture, Black will have already won the knight b3, and threaten Bxd1 and Qc5+.

Sometimes capturing the defender can involve sacrifices and other motifs, as in the following example:

White has just played Nd5, attacking the pinned knight f6, but this was a terrible mistake.
Black won with the surprising Nxd5, capturing the knight which was defending e3, and attacking it with the knight.
Black moved the knight out of the pin, but if White captures the queen with Bxd8, Black can play the knight fork Ne3+, and win the queen back with Nxd1, winning a knight at the end.

Visualization

When you are threatening a fork, but the forking 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 defender.