Explanation

Protecting a square is insufficient, if the protector can be forced away, because it has to capture somewhere else, leaving the square unprotected.

Sometimes a protector has more than one task. It has to protect a piece and a square, or two squares. Such a protector is called "overloaded", if after performing one of the tasks, it is deflected away from doing the other.

Examples

The white bishop on g2 is protecting the knight on h3, and the square f3.
It is overloaded, and Black can win with the deflection Bxh3.
After White recaptures with Bxh3, the bishop is deflected away from protecting f3, and Black can win with the royal fork Nf3+, giving check and attacking the queen at the same time.

Sometimes a deflection can involve a temporary sacrifice, as in the following example:

The white rook on e2 is protecting the bishop on b2, and the square e3.
It is overloaded, and Black can win with the deflection Qxb2, temporarily sacrificing queen for bishop.
After the rook recaptures with Rxb2, it is deflected away from protecting e3, and Black can win with the royal fork Ne3+, giving check and attacking the queen.
The investment was a good one, in the end Black has won the bishop on b2, and is a knight up in the endgame.

Sometimes deflections can be combined with additional motifs:

The queen on d7 is protecting the knight on d5, and the square e7.
White can win with Qxd5+, combining decoy and deflection.
After the queen recaptures with Qxd5, it is deflected away from protecting e7, and also decoyed to d5.
White can then win the queen back with the royal fork Ne7+, having won a knight in the end.
This combination of decoy and deflection happens quite frequently, even in games of strong players.

Visualization

When you are threatening a fork, but the forking square is protected, always think about ways to remove the defence.
If a protector has more than one task, always check if a deflection is possible.

Related combinations

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