Explanation

Protecting a square is insufficient, if the protector can be forced away with an attack, leaving the square unprotected.

Examples

Black just attacked White's queen with Rb6.
Qd7+ would fork Black's king and rook, however the king is defending d7.
White can win by attacking the defender with Rc5+!
Black has to play Kb7, leaving d7 unprotected.
White can now win a rook with the queen fork Qd7+.

Sometimes attacking the defender can be combined with other motifs:

Black can win with Bf5!
The bishop attacks the rook, which is defending square e3.
The bishop also clears the way for the rook on e7 to e3.
After White runs away with the rook, for example with Rxd6, Black can win a bishop with the rook fork Re3+.

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.