Sacrifice or Kamikaze, Strategy or Tactics?

A recent Timeforchess.com game of mine began

1. d4 Nf6 2. c4 g6 3. Nf3 Bg7 4. Nc3 O-O 5. g3 d6 6. Bg2 c5 7. O-O Na6 8. Bg5 h6 9. Bxf6 Bxf6 10. Qd2 Bg7 11. Rd1 Be6 12. d5 Bd7 13. e4 Nc7 14. Nh4 b5 15. cxb5 Nxb5 16. Nxb5 Bxb5 17. Re1 Rb8 18. b3 a5 19. Bf1 Bxf1 20. Rxf1

Qxh7 vs. TimeforChess Opponent

Black to Move

This is a very instructional game to me. The game continues

20. .... a4

21. bxa4 Rb2

22. Qd3 Qa8

23. Rd2 c4

24. Qe2 c3

25. Rc2 Qxa4

26. Rfc1 Rfa8

27. Kg2

Black to Move

And white's game looks hopeless to me. a4 was a subtle dagger that lead to all of this. Granted my knight is misplaced on the edge there at h4, but beyond that it was a very competitive game. Suddenly a quiet little "sacrifice" of blacks isolated a-pawn and my world comes crashing down.

Color me impressed. Its something I will certainly remember. I'm not certain I understand it enough to make use of it in my own games though. Was a4 a tactical manuever, or strategic? I'm not sure. It certainly lead to a windfall strategic position for black. Who would be more impressed by a4, Silman or MDLM?

The game finished

27. Kg2 Qb4 28. Nf3 Ra8xa2 29. Ne1 Kh7 30. f3 Qb3 31. Kh3 e6 32. Qd3 exd5 33. exd5 Qb6 34. f4 Qd4 35. Qf3 Qd2 36. Qg2 Rb2xc2 37. Rc1xc2 Qd1 38. Rc2xa2 Qh5 0-1

I overlooked a simple mate in 1 at the end, mostly because I had been preoccupied with the passed pawn for over a month of real time (thats correspondence chess for you) and when I finally saw a chance to eliminate some of that pressure I just jumped and made a move immediately.

C'est la vie!