Deflection is a psychological term used to describe a coping mechanism that people use to avoid dealing with troubling feelings or situations. People do this by being able to alternate from one topic to another so that they do not have to deal with either topic in depth. Ok, so what does this have to do with chess? In chess deflection will win you games plain and simple. Players will often use a piece, especially queens, to guard or protect two separate pieces at the same time. This is where the term deflection comes in.
Also sometimes called an overworked piece, deflection is a tactical theme in which one player has a piece that must remain on a square that it is on, either because it defends another piece, or because it blocks a threat. If this guarding piece were forced to move, the opponent's position would crumble. The reason it is sometimes refers to as the overworked piece is because this defending piece is forced to do too many things at once, defending two pieces. By meeting one threat, the other threat is left unattended to and thus leaves this piece at its opponent's mercy.
For example, if a bishop is guarding a queen and is in place to prevent a back row mate from a rook, you can simply move the rook to the opponent's back row and place the king in check. This forces the bishop to move to block the check leaving the queen that the bishop was defending completely unguarded and available for the taking. Seek out deflection that your opponent is using and exploit it. This becomes an even better tactic towards the end game because the chances of your opponent having a single piece defending two is much higher.