Stockfish started game 21 with an eval advantage than stayed below 1 for a long time. The position was open and the engines exchanged pieces gradually. The game reached a QN vs QN position on move 40 without anything exciting happening, and the expected outcome was a draw. On move 48 Houdini moved a pawn, Stockfish's PV recommendation was to start walking the king towards the center.
Stockfish's eval instantly jumped over 2 and then 3 as it forced a queen exchange. Houdini continued to move its king to the center, realization that it was in trouble came two moves too late. The knight ending was far from trivial but both engines agreed that Stockfish was winning.
The black king had to choose a side, it chose the queen side and Stockfish captured two pawns on the king side before the game was adjudicated. After reading the comments on the chat it is still not clear whether g6 was a blunder or whether Houdini would have lost in any case. The win was not obvious and hard to spot in advance.
Stockfish offered a rook for a bishop in game 22. Surprisingly the game became drawish very quickly. Houdini's king was exposed to attacks, Stockfish used this to capture a pawn. Eventually the engines exchanged pieces reaching a R vs B drawn ending.
Game 23 was a King's Gambit, always an interesting opening to watch. In this variant black had two advanced pawns on the king side, facing white's exposed uncastled king. Stockfish's defense was to sacrifice a knight to eliminate the pawns, leaving the king side open. Stockfish had more space and its pieces were more active on the king side despite being a piece down. Houdini's eval remained over 0.5 for a while, after exchanging most pieces evals came down to 0. The game reached a pawns vs knight tablebase draw.
In game 24 Stockfish kept its king side pawns by trading the white knight for a bishop as soon as it could. The black queen side was undeveloped, and on move 11 Stockfish offered a whole rook with the simple Nxc7 fork. Analysis in the chat suggested that the black counter was g3. Houdini thought for a long time about how to continue.
After 20 minutes Houdini decided against taking the rook. The position gradually stabilized. Houdini was a pawn down, Stockfish let go of the f pawn but still had a pawn majority on the king side. Both kings left the exposed king side, and neither was in danger. Houdini had a bishop pair, though not very active. Stockfish had an eval of 1.5, but it only fluctuated without increasing.
Stockfish's eval started to increase, it wasn't clear why at first. Stockfish found a great square for its knight at f3, hitting the h pawn and restricting the white rook. The black king moved off the back rank to avoid check threats and the white d pawn blocked any attempts by Houdini to attack on the long diagonal.
Houdini tried to apply pressure on the black g pawn, and then there was a RB for Q trade and Houdini added a bishop for pawns sacrifice to expose the black king to checks. The Q vs RBN position was hopeless for white, the game was adjudicated a few moves later. An incredible win for Stockfish, very difficult to really understand. Houdini with 5 losses in 24 games, seems to be much weaker than Stockfish in this superfinal.
Evals stayed close to 0 in game 25, the engines exchanged pieces until reaching a BN vs BN drawn position. Game 26 started with low evals as well. Most pawns remained on the board and the engines shuffled behind the pawn lines. On move 39 Houdini gave a pawn on the queen side and opened a file.
Houdini couldn't get the pawn back, moreover it was Stockfish that took over the open file. Stockfish's eval was over 1 when it gave the pawn back, creating a passer that could be backed from behind.
Houdini tried to counter on the king side, opening up files and threatening the black king. As a result its own king was exposed and Stockfish could use the same open files for an attack of its own. There were two cases of 2-fold repetition where Houdini's eval dropped to 0, Stockfish made up its mind each time to avoid a draw and its evals were over 2.
Stockfish exchanged bishops and started to capture the white pawns. Houdini's position crumbled and Stockfish was soon 3 pawns up. Houdini finally captured the queen side passer, exchanging queens as well, and the game was adjudicated just before Stockfish won a piece. The third opening in a row that Stockfish wins one of the two games and draws the second.
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