A protected passed pawn is a passed pawn defended by another friendly pawn, which makes it immune to capture by the enemy king.
When a passed pawn is guarded by a pawn beside it, it gains a superpower: the defending king can never take it. Capturing it would mean stepping onto a square controlled by the guard pawn, which is illegal for a king. Pieces that capture it are simply won by the recapture.
The result is permanent paralysis. The defending king must stay inside the square of the passer forever, since no other unit can babysit it as cheaply, and that standing duty means the king cannot fight for anything else on the board. You can maneuver at leisure, create a second weakness, or invade on the other wing.
In pure king and pawn endgames a protected passed pawn usually beats an outside passed pawn: the outside passer can at least be rounded up by the king, while the protected passer cannot be touched at all. Just watch for one drawing trick: if the defending king can blockade the pawn on the square directly in front, some positions turn into fortresses.
Because the square the pawn stands on is covered by the friendly pawn behind it. A king may never move onto a square attacked by an enemy unit, so the capture is illegal, not just risky.
No. It is a large, durable advantage, but the defender can sometimes hold by blockading the square in front of the pawn with the king and setting up a fortress. It wins games by fixing the enemy king while you create a second front.
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