21 Apr '11 18:42>2 edits
position after 16. ... RxNc3
(From Hartston-Westerinen, 1966- but it's been played a bunch of times!)
Andrew Martin annotates this position from Starting Out: The Sicilian Dragon
"The exchange sacrifice is a key resource for Black and we will see it cropping up time and again throughout this book, while Black achieves a fair amount with it:
1) He demolishes White's pawn structure.
2) He weakens White's king position.
3) There is the distinct possibility of rounding up White's isolated pawns on the queenside.
4) The potential threat of Nd5 is removed and so the black knight will be much safer on f6, protecting h7 against possible checkmate.
5) He can now start to think about his own attack, rather than submissively defending and hoping for the best.
6) Endgames can be quite favourable for Black as the white rooks tend to struggle to make an impression against Black's watertight pawn structure.
All in all not bad value!"
It is generally recognized that the two center pawns and better structure give black the better ending, and it's a testament to the quality of the position that an exchange sac is so effective as to be considered almost routine.