After the pawn sacrifice, I wrote, "Ivanchuk appears to have compensation for the pawn, but no significant advantage." Looking through the game this morning, I reached a different conclusion. Karjakin's gain of a pawn left his pawn structure in a mess. All his queenside pawns became weak. Although the a- and c-pawns are not technically isolated because the b-pawn remains on the board, they are too far advanced to receive assistance from the backwards b-pawn. Moreover, White's bishops control all the key squares. Over the course of the game, Ivanchuk's bishop pair mopped up Karjakin's weakened pawns. Eventually, Ivanchuk was the one with an extra pawn and was ready to go two pawns ahead when Karjakin threw in the towel. Karjakin went on to win the tournament with five wins, six draws, and two losses; Ivanchuk finished in a share of last place.
On the other hand, my engine finds the position equal after Ivanchuk's sacrifice.
The annual tournament in Wijk aan Zee was begun as a local event in 1938 by the Hoogoven Chess Club, consisting of workers at the steelworks. In 1939 it attracted national attention, and then in subsequent years gained international attention. It has grown into one of the major annual Grandmaster tournaments and a chess festival for all levels of players. As the local steel factory has changed ownership, so has the name of the tournament. In 2007, India's Tata Steel purchased Corus, and so beginning with the 2011 Wijk aan Zee event, it is now called the Tata Steel Chess Tournament. One of the first chess books that I purchased and studied regularly was Wijk aan Zee: Grandmaster Chess Tournament 1975 (1976). This old paperback is showing its age from many years of use, but remains a treasured possession.
White to move
White's pawn on b4 is pinned to the queen by Black's bishop on e7. However, the bishop is undefended and b4-b5 attacks the Black queen. Thus, the pinned pawn's advance creates a discovered attack on the bishop, and Black has no choice better than exchanging bishop for knight.
23...axb5 24.Qxe7 bxa4 25.Rd1 Nf8 26.Rd6 Re8 27.Rxc6 Rxe7 28.Bc5
Black to move
Houdini sees a difference of just under half a pawn between the move Karjakin played here and the optimum choice. The Chess Informant annotations provided by Ivanchuk and the lines suggested by Houdini both favor 28...Re8. This move has the idea of swapping rooks before other material comes off.
Karjakin played 28...Rd7. He was under considerable time pressure, having thought long before playing 22...f5.
29.Rb6 fxe4 30.fxe4 c3 31.Kf2 Ba2 32.a6 bxa6
White to move
Black is temporarily ahead by two pawns, but positionally, White has a clear advantage.
33.Rb8 Rf7+ 34.Ke3 g6 35.Bd6 Rf6 36.Rd8
Black to move
Karjakin is not in zugzwang, but almost. There is nothing useful that he can do. I revised my analysis of the position after Ivanchuk's pawn sacrifice because even with optimum play, there is little that Black can do other than watch White build up his attack against the weakened pawns. With the rooks off the board, perhaps the technique would have required more skill. It seems to me that this sort of position merits one of those comments, "the rest is a matter of technique." Such comments seem lazy, of course, and are spurned by chess readers. For a super-grandmaster like Vasily Ivanchuk, such technique has been honed through extensive preparation, lots of playing experience, and keen instincts. For the rest of us, such technique requires that we continue to learn. Even so, White's plan is simple and straightforward: mop up the pawns and turn the remainder of the endgame into one where White has more.
When it is clear that White has more pawns, Black can resign in a grandmaster game. Among class players, resignation is deferred until the pawn promotes. Among young scholastic players, checkmate must be the finish.
36...a3 37.Bxa3 Kg7 38.Bd6 Rf7 39.Bxe5+ Kh6 40.Bxa6 Ne6 41.Rc8
Black to move
Karjakin resigned in this position. Another time to resign for a grandmaster is after reaching the time control. Karjakin played a lot of moves in a hurry. Now, with time to consider his position, he sees clearly that he is sunk.