Examples

White's queen has to protect two squares: the promotion square f1, and e4 against checkmate.
The queen is overloaded, and Black can win with Be4+!
After the forced reply Qxe4, the queen is deflected away from protecting the promotion square.
Black can promote with f1=Q#, which is also checkmate.
Note that f1=Q first would not have worked, as after Qxf1 Be4+ is not checkmate.

In this rook endgame, White has a dangerous passed pawn on a7.
The rook on a6 is x-ray protecting the promotion square.
White can win with the surprising deflection Rb6+, giving check and attacking the rook at the same time.
After the forced reply Rxb6, the rook is deflected away from x-ray protecting the promotion square, and White can promote and keep the new queen.
This type of deflection is very common in rook endgames.

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 deflection, other ways to remove the defence include capturing the defender, and attacking the blocker.