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 bishop on h6 guards all the escape squares of the king on g8.
The white knight on d5 could deliver checkmate on e7 or f6, but both squares are protected by the queen on d8.
The queen also protects the square e8 against mate from the rook on e1, therefore it is overloaded.
White can win with the deflection Re8+.
After the forced reply Qxe8, the queen is no longer defending f6, and white can checkmate with Nxf6#.
Note that starting with Nxf6 does not work, as after Qxf6 the square e8 is now guarded by the rook on a8.

The rook on e8 is guarding the square f8 against Qf8#, and the square e7 against Ne7#.
It is overloaded, and White can force checkmate with the deflection Qf8+.
After the forced reply Rxf8, the rook is deflected away from protecting e7, and White can deliver checkmate with Ne7#.

Visualization

When you are threatening checkmate, but the checkmate 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.