hikaru vs Ykow2

White pushed the b‑pawn two squares (b2‑b4). The move attacks the black knight on a5, removes the only undefended white pawn (b2), and creates a new pawn on b4 that controls a5 and c5. By doing so White eliminates the immediate danger of the knight hopping into c4 or b3, and simultaneously prepares to open the b‑file for the rooks. After the push the material balance is unchanged, but White now has a concrete target on a5 and the black king’s only undefended piece (h7) remains a liability for Black.

The engine’s top line after 24.b4 is 24…Kh8, indicating that Black’s best reply is simply to tuck the king away; any more aggressive try (e.g., 24…a3 or 24…e4) would lose material because the b‑pawn now attacks the a5‑knight. If White had played a quieter move, Black could continue with ...e4 or ...a3, exploiting the undefended white pawn on b2 and the weak g4‑queen. By playing b4, White forces Black into a passive king move and gains a tempo on the a5‑knight, turning an opponent’s active piece into a target. This is why the engine rates the move as the best – it creates a direct threat while neutralising Black’s most dangerous ideas.

Create Immediate Threats While Neutralising Opponent’s Threats: A well‑timed pawn push can both attack an enemy piece and solve a defensive problem (here, the undefended b2 pawn). By forcing the opponent to respond passively, you gain the initiative and dictate the flow of the game.