context: 1. g1f3  1... d7d5  2. e2e3  2... g8f6  3. c2c4  3... e7e6  4. b1c3  4... c7c6  5. b2b3  5... b8d7  6. c1b2  6... f8d6  7. d2d4  7... e8g8  8. f1e2  8... b7b6  9. e1g1  9... c8b7  10. d1c2  10... f8e8 In these positions from the Semi-Slav Defense there's always a question about how to develop the heavy pieces, since several configurations are possible, especially for the rooks. The most common development is with the queen on e7 and the rooks on c8 and d8, supporting a possible c6-c5. 11. a1d1  11... d8c7 The queen takes an alternative route. 12. h2h3  12... a7a6 Preparing c6-c5 without White having Nb5. Another plan is Rad8-Qb8. 13. f1e1 This doesn't seem like the scariest of moves, but the problem for White in this line is that the e3-e4 advance isn't that promising and patience is needed. 13... c6c5 A new and natural move. 14. c4d5  14... e6d5  15. d4c5  15... b6c5 Hanging pawns on the board. This typical pawn structure is probably the second most important in chess, after the isolated queen's pawn. 16. f3h4 White prepares an active plan. This is a bit risky, but has good practical value for a blitz game. 16... d7e5  17. h4f5  17... d6f8  18. f2f4 A bold decision. 18... e5d7  19. e2f3  19... g7g6 The losing mistake. Black has two acceptable alternatives. 20. c3d5 This isn't a sacrifice, just a tactical operation that wins a pawn. 20... f6d5  21. f3d5  21... b7d5  22. d1d5  22... c7c6  23. c2c3  23... f7f6  24. e1d1 