Tactics & checkmates
Overloading
A piece is overloaded when it has several defensive duties that it cannot continue to perform after a concrete threat. Merely defending two things is not enough: those duties must conflict tactically.
A practical example
A rook protects an otherwise undefended queen along its rank and a mating square along its file. An attack can force a choice: save the queen and abandon the mating square, or prevent mate and lose the queen.
A common mistake
Attacking an apparently overloaded piece without calculating the move order. A forcing exchange or countercheck may give it time to solve both problems.
Try this in your next review
Find defenders with multiple duties. Play out the first threat mentally and identify which protection actually disappears. Check whether another defender can take over.