What is prophylaxis? In simple terms it is prevention. The idea of chess prophylaxis is to stop your opponents plans before proceeding with your own. To paraphrase Nimzovich, "Do not always be thinking of attack! Moves that safeguard your position are often far more prudent."
A good explanation is given by IM Silman in "The Complete Book of Chess Strategies," "Prophylactic play can be very subtle or brutally straightforward. The position in the following diagram gives us a tast of both: the answer is quite straightforward but also suprising and, therefore, contains some subtely."
White has a substantial advantage in space and piece activity. Usually black tries to free himself in these positions via ...e6-e5 or ...c6-c5. White has already cramped down on the e5 square, however black is about to play ...c6-c5 a move that gains space, hits at whites center and prepares to develop the b7-bishop. What should white do about this? Prophylaxis to the rescue!
1. c5! clamps down on black's expansion and grabs even more space. True, black has access to the d5 square but one square for one piece cannot make up for the asphyxiation of an entire enemy. By following up with Ne5 white can build a kingside attack at his leisure while Black can only stare helplessly and wait for his doom to arrive."
The following game is a fine example of prophylactic understanding:
1. d4 Nf6 2. Nf3 e6 3. e3 b6 4. Bd3 Bb7 5. O-O Ne4 6. Nbd2 f5 7. c3 Be7 8. Qc2 d5 9. Ne5 O-O 10. f3 Nxd2 11. Bxd2 Nd7 12. Nxd7 Qxd7 13. Rae1 c5 14. Qd1 Rf6 15. Qe2 Raf8 16. Bb5 Qc7 17. f4 c4 18. Kh1 Bd6 19. Rf3 Black pawns point to thwe queen-side which suggest queenside play. Black has prospects with the plan of ...a6-b5-b4 and opening lines on the queenside. This looks immediatley playable but Capablanca shows he has a deeper understanding of the position. Whites only counter-plan consists of a break based on the advance g4 in order to activate his rooks on the g-file and his bishop on c2. Capablanca instead of going forward with his attack squelches all possible play by his opponent: this is the truest possible meaning of prophylaxis 19... h5! 20. Ref1 Rh6! Nimzovich commenting says, "black sees whites h3 and g4 coming, and he wants to be ready to attack on the h-file when this happens." Once whites kingside play fails his position crumbles. 21. Be1 g6 22. Bh4 Kf7 23. Qe1 a6 24. Ba4 b5 25. Bd1 Bc6 26. Rh3 a5 27. Bg5 Rhh8 28. Qh4 b4 29. Qe1 Rb8 30. Rhf3 a4 31. R3f2 a3 the dam is breached 32. b3 cxb3 33. Bxb3 Bb5 34. Rg1 Qxc3 35. Qxc3 bxc3 36. Rc2 Rhc8 37. Bh4 Bd3 38. Rcc1 Rxb3 39. axb3 a2 0-1 Kupchik-Capablanca, Lake Hopatcong 1926
Often when it appears that there is not much one can do in a given position prophylaxis- the preventing of your opponents plans- is a logical recourse as in the next example:
1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.e5 Qd7 5.Bd2 b6 6.Nf3 Bf8 7.Be2 Ba6 8.O-O Ne7 9.Bxa6 Nxa6 10.Qe2 Nb8 11.Nd1 c5 12.dxc5 bxc5 13.c4 d4 14.Ne1 white has a significant lead in development and plans a kingside attack with f4, g4 and an eventual f5. Blacks protected passed pawn is useless at the moment and he has no active plan. What do do? 14...h5! Prophylaxis! Since black has no active counterplay he plays to stop his opponents plan 15.Nd3 Nf5 By using his h-pawn to restrain blacks g4 advance he has now obtained a good aquare for his knight. 16.f4? Not a good idea as this hems in whites pieces 16...Qb7 17.N1f2 Nd7 18.Qe4 Qxe4 19.Nxe4 a5! More prophylaxis as this prevents b2-b4 and if 20. a3 then 20...a4! further restrics white 20.g3 Be7 21.Kf2 Nh6 White intends to control f5 by means of h3 and g4 so black plays to stop this plan. More prophylaxis! 22.h3 f5 23.exf6 gxf6 24.Rae1 Kf7 25.Ke2 Rhg8! having stopped white g-pawn from advancing black now treats it as a weakness 26.Kd1 Nf5 27.Rg1 Nb6! taking aim at white Q-side pawns as well 28.b3 a4 29.Ndxc5 axb3 30.axb3 Nxg3 31.Rxg3 Rxg3 32.Nxg3 Bxc5 33.f5 exf5 34.Nxf5 d3 35.Bc3 Ra3 36.Kd2 Rxb3 37.Ra1 Bb4 38.Ra7+ Kg6 39.Ne7+ Kg5 40.Bxb4 Nxc4+ 41.Ke1 Rxb4 42.Rd7 Rb1+ 43.Kf2 Rb2+ 44.Kg1 d2 0-1 Timman-Seirawn, Lone Pine 1978
Here is an example of prophylactic play from one of Nimzovich's own games that appears completely modern:
1. Nf3 Nf6 2. d4 d5 3. c4 e6 4. Nc3 Be7 5. e3 O-O 6. a3 a6 7. c5 c6 8. b4 Nbd7 9. Bb2 Qc7 10. Qc2 e5 11. O-O-O e4 White by playing an early b4 allowed black to break with ...e5 and now black having played ...e4 is looking for a kingside attack by ...f5-f4. Against this Nimzovich found an excellent prophylactic idea: 12. Nh4! threatening Ne5 12...Nb8 13. g3 Ne8 14. Ng2 f5 15. h4! In his pamphlet, Blockade, Nimzovich comments, "And blacks kingside which was seemingly ready to march, is paralyzed." 15..Bd8 (15...g6 [15...h6 16. h5] 16. a4 h6 17. Nf4! Kg7 18. h5 g5 19. Ng6 with an overwhelming advantage for white) 16. a4 b6 17. b5! Nf6 18. Nf4 axb5 19. axb5 Qf7 20. Be2 Bc7 21. cxb6 Bxf4 22. gxf4 Bd7 23. Kd2 cxb5 24. Ra1 Nc6 25. Bxb5 Na5 26. Be2 Rfb8 27. Na4 Bxa4 28. Rxa4 Rxb6 29. Bc3 Nb3+ 30. Qxb3 Rxb3 31. Rxa8+ Ne8 32. Bd1 Rxc3 33. Kxc3 Qc7+ 34. Kd2 Kf7 35. Bh5+ g6 36. Rha1 Qb6 37. Be2 Kg7 38. Ke1 Nc7 39. R8a5 Kh6 40. Kf1 Qb3 41. h5 Ne8 42. Ra6 Qb2 43. hxg6 hxg6 44. R6a2 Qb7 45. Ra7 Qb2 46. Kg2 Nf6 47. Rh1+ Nh5 48. Bxh5 gxh5 49. Rha1 1-0 Nimzovitch-Bernstein, Karlsbad 1923
One of the great propitiators of modern prophylactic play is Anatoly Karpov here is a fine example of prohylaxis from a Karpovian Ruy Lopez:
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4 6. d4 b5 7. Bb3 d5 8. dxe5 Be6 9. c3 Bc5 10. Nbd2 O-O 11. Bc2 Bf5 12. Nb3 Bg6 13. Nfd4 Bxd4 14. cxd4 a5 15. Be3 a4 16. Nd2 a3 17. Nxe4 axb2 18. Rb1 Bxe4 19. Rxb2 Qd7 20. Bd3 Bxd3 21. Qxd3 Rfb8 white could now try to launch an attack with f4, but black is threatening counterplay by ...Na5-c4 so he plays to prevent this. 22. Rfb1 b4 23. h3! Yusopov notes that this not only prevents the active ...Qg4, but it also creates a useful escape square 23...h6 24. Rc1! More prophylaxis. Now on 24...Na5 white has 25. Qb1! with the idea of 25...Nc4 26. Rxb4 Rab4 27. Qxb4 Rxa2?? 28. Qb8+ Kh7 29. Qb1+ 24...Rb6 25. Qb1 Rab8 26. Rc5 Nd8 27. Rcc2! More prophylaxis stopping...Nb7-a5 27...Nc6 28. Qc1 R8b7 29. Rc5 More prophylaxis again stopping...Na5. As Yusopov says, "When you are not permitted to carry out your main idea it is very difficult to carry on the fight." 29...Ne7 30. Kh2 Nf5? this loses a pawn 31. Rbc2 Rg6 32. Rxc7 Rxc7 33. Rxc7 Qb5 34. g4 Nh4 35. Rc8+ (35. Qc5! wins by force) 35...Kh7 36. Qd1 Qa6 37. Rc2 f5 38. Kg3 fxg4 39. Kxh4 gxh3 40. f4 Qe6 41. Qh5 Qe7+ 42. Kxh3 Qf7 43. Rh2 Qd7+ 44. f5 1-0 Karpov-Yusopov, Moscow USSR Ch. 1983
Here is a final test position taken from "Winning Chess Strategies" by GM Seirawan (diagram below)
It is black to move. What should he play?
Black has 2 well posted knights. The knight on c5 is safe enough because if white plays b4 he will weaken the c4 square which black can make use of by the maneuver ...Nd7-b6-c4, and if he exchanges with Bxc5 after ....Bxc5 the black dark squared bishop has a very nice diagonal. What black needs to do is ensure the safety of his other strongly posted knight by securing the f5 square with 1...h5!. When you control an important square make sure it remains in your hands- GM Seirawan. If black fails to play prophylactically here and routinely develops with say 1...Be7 then white gets the advantage by 2. g4! Nh6 3. h3 and blacks once proud knight on f5 is miserable and doing nothing on h6.