Removal of the guard
Removal of the guard is capturing or driving off the piece that defends a target, so the target can be taken.
If a bishop is only safe because a knight defends it, taking the knight leaves the bishop hanging. The same idea works against a mate-square defender: remove the guard, then mate.
The knight on c6 is the only protection for the bishop on d5. Qxc6 takes the guard. After that, d5 is no longer defended and the bishop can be collected.
Count defenders before you capture. If you and the opponent have the same number of attackers and defenders, removing one guard tips the trade.