Removing the defender

You counted. The bishop is attacked once and defended once, so it is safe. Then a move later it is gone, and the engine says you lost a piece four moves ago.

What happened is that safety was borrowed. A defended piece is not safe, it is conditionally safe: safe for exactly as long as the defender stays where it is and stays able to do its job. Attack the defender instead of the piece, and the condition breaks.

Three ways to break it, in the order they show up in real games: take the defender, chase the defender, or give the defender a second job.

Take the defender

Nxe5 straight away is nothing: Nxe5 Nxe5, a knight for a knight. But the defender is a knight and White has a bishop pointing at it. 1.Bxc6 bxc6 2.Nxe5 and the pawn is White's, because the piece holding e5 no longer exists.

This is the whole Exchange Variation of the Ruy Lopez in one line, and it is also the single most common way a pawn or a piece changes hands between 1000 and 1800. Nobody blunders it in one move. They blunder it by counting attackers and defenders once and never counting again.

Chase the defender

You do not have to capture the guard. Making it move is enough, and a pawn does it for free.

1.b5 and Black is asked a question with no good answer. Move the knight and the bishop hangs to Rxe5. Move the bishop and the knight hangs to bxc6. There is no knight move that keeps guarding e5, and there is no bishop move that starts guarding c6. White wins a piece.

Notice what the pawn move did not do: it did not attack anything valuable, and it did not threaten mate. It just made the defender's position untenable. Pawn moves that hit a defending knight are worth looking for every single game, in both directions.

The defender of a square

The guard is not always guarding a piece. Often it is guarding a square, and back-rank mates are what happens when it stops.

Black's rook has one job it does not know it has. White offers a rook on e8, the offer cannot be declined (it is check, and the king has no square), and the second rook delivers mate on the square the defender used to hold. The material sacrifice is real and it is irrelevant, because the point was never material.

This is why “two attackers against one defender” is a counting rule and not a guideline. When your attackers outnumber the defenders of a square near your opponent's king, the defenders can usually be removed by force.

The tell

Look at your own pieces and find the ones whose safety depends on exactly one defender. Those are the ones that fall. Then ask two questions about each defender:

1. Can it be taken? By anything, even at the cost of a trade.
2. Can it be hit by a pawn? Pawns are the cheapest way to move a piece that does not want to move.

And a third, which is the one people skip: is the defender already busy? A piece defending two things is defending neither, and a piece that is pinned is defending nothing at all.

The habit

The counting habit from hanging pieces has a second half. First count attackers and defenders. Then look at the defenders themselves and count their attackers. That is the move ahead almost nobody at club level takes, and it is where the tactic lives.

The reverse is your best source of free material. Every time you are looking for something to play and nothing suggests itself, find the enemy piece that is doing defensive work and ask what happens if it disappears.

Then go and look at what it cost you. On this site, filter your games by hanging a piece and losing the exchange, then step backwards two moves from the mistake. In a good share of them the piece was defended when you moved it, and the defender was the thing your opponent attacked.