I don't understand something

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Gaudalf

Hello, can someone explain to me why Rb4 was the best in move 51? It's my third match ever here, I used to play a bit of chess before, but my main problem is endgames. It always has been.

I look forward to your feedback. Here's the link:

https://www.chess.com/game/live/139806983782

bigD521

I suggest that it keeps the King on the a file, which blocks the pawn from promotion.

GMegasDoux

In this end game there are a king rook and pawns vs king rook and pawns. The goal is to get a material advantage by promoting pawns. Here, black requires pieces to protect the outside passed pawn to promotion. The king must be within 1 square to do this so the rook can keep the king tied to the pawn by blocking the b file or attacking the pawn directly. The other side has pawns in conflict with a darting rook vs a king which can move around the pawn structure. Again white is aiming for a passed pawn, but this will not be on the side or the board. With best play the computer indicated that the final pawn will be the black h pawn, if that comes off the position will be a draw. There is something called the Lucaina position where a king rook and 1 pawn can defeat a king and a rook. The defending side must attack the pawn and king from behind, but the attacking side must cut off the defending king from the pawn that is promoting and then the attacking king needs to strafe between files before using the rook to protect it from check. This allows the final step in promotion and clears both rooks from the board. It does not seem clear in this end game as "building a bridge" (what the maneauver is called) is when you have the pawn not on the a or h files.

GMegasDoux

Although, I have not had a position to attempt that on the edge of the board so I can not be certain if you can build a bridge from the edge files. I can not recall it being said in theory, but I have only seen it illustrated on central files.

zone_chess

Rb4 cuts off the king, and that way his most valuable pawn has been stopped from promoting. This is a decisive advantage.

You also missed Qc3 on move 23, with the double threat of capturing the knight and pinning the queen to the king with the bishop.