hikaru vs Ykow2

White chose 20. gxf6, letting the pawn on g5 capture the black pawn on f6. The capture creates a passed pawn on f6 but simultaneously vacates the g5‑square, leaves the h5 pawn completely undefended, and does nothing to address the black queen’s threats on a2, c3, e4 and the looming g5‑square. Moreover, the white bishop on e3 remains undefended (white_undefended: e3) and can be targeted by Black’s pieces. The move therefore trades a pawn for a pawn without gaining material or improving the position, while exposing new tactical liabilities.

The engine’s 20. Bxc5 exploits the immediate tactical opportunity: the white bishop on e3 captures the black knight on c5, winning a piece. By removing the knight, White eliminates a key defender of the d6‑square and reduces Black’s pressure on the queenside (the queen’s a5‑a2 and a5‑c3 ideas). After Bxc5, White still retains the pawn on g5 to protect h5, keeps the bishop on c5 active, and preserves the pawn structure around the king. In contrast, 20. gxf6 allows Black to continue with ...Qxf6 or ...Bxe3, regaining the pawn and potentially winning material. The engine’s move therefore secures a clear material advantage and avoids the creation of new weaknesses.

Capture the free piece, don’t create weaknesses: When an opponent leaves a piece unprotected, the priority is to win that piece (or the equivalent material) rather than launching a pawn break that leaves your own pieces undefended. Material gain and piece safety outweigh speculative pawn storms.