As black you must know a lot of opening theory, or at least the basic procedures on how to continue in a large variety of settings. You can't just higgledy-piggledy play any old thing. One structure he covers in some detail is the Scheveningen/Najdorf/Classical.
Important points to note in these setups:
Fluid P-structure - may become more stable if white answers ...d5 with e5. If black exchanges off his his e-Pawn for white's e-Pawn the result will be isolated Ps for both sides.
White has a space advantage so has more room to operate. For example, just comparing the light squared Bs, white can choose between c4, d3, e2 and b5. Black's can only go to e7.
Development - Black has made a lot of P moves and will likely also throw in ...a6. All the while he's doing that white has been developing. Dr. Tarrasch would not approve of black's play!
White has a lot of freedom for operations on the K-side - except for the N on f6, no black pieces have any influence there.
These factors give white cause to be optimistic about his K-side attack which he can carry out in several ways. White's most dangerous plan is to play e4-e5 which opens the f-file, the b1-h7 and c1-h6 diagonals and it also gives him the square e4 which allows him to transfer the N. In the process black's lone N defender will be driven away. White has a number of ways he can execute this plan. The Richter-Rauzer (Bg5), the Sozin (Bc4), the Classical (Be2 and Bd3 or even Bf3).
Another white option is f4-f5. This puts pressure on the e-Pawn and if black plays ...e5, white has the d5 square for his N. This plan is especially good if white develops his light squared B on the a2-g8 diagonal. Under the right circumstances white can advance his h-Pawn. The possibility of piece sacrifices on b5, e6, d5 and f5 abound.
White can also advance his g-Pawn and when he pays g4-g5 it drives away the N on f6 and so white has two main avenues of approach: a) play f4-f5-f6 or b) mass his heavy pieces on the h-file. Statistically white wins the short games, black wins the long ones.
As black, your defense must be accurate! All pretty off putting if you want to play the Sicilian as black!
Whole books have been written on various Sicilian lines, so because he was not writing an opening book, Yermolinsky concentrates only on the option where white chooses to advance the g-Pawn. When white plays this line, what does black do? He must counterattack! One of white's major mistakes in this variation is overconfidence which often causes him to launch his K-side attack before completing his development. This may give black a chance to sacrifice a piece and seize the initiative.
Here is an exciting and instructive game Yermolinsky played against Fedorov in the Leningrad Championship back in 1985.