Everybody knows that you can't win by just marking time, you have DO something. For a long time it was believed that having the first move automatically gave white the advantage while black had to play to equalize. Thanks mostly to investigation and practice by leading Soviet players of years gone by it became the norm for both white and black to fight for the initiative.
In modern practice you won't find a lot of QGD Orthodox Defenses even though in Capablanca's day it was just about all you did see. Even the great Alekhine had a low opinion of moves like 1...g3 for black. It wasn't until the 1950s that Soviet players began playing the King's Indian, for example, and they proved that black had a lot of opportunities for active counterplay that enabled him to seize the initiative for himself. The Sicilian also became a potent weapon for black for the same reason.
Today black is no longer content playing to neutralize white's initiative by exchanging and simplifying; he fights for the initiative and it's this fight that is the modern criterion for determining the worth of any opening system.
Sacrifices for the initiative are common and in many cases your opponent isn't forced to play any particular moves or follow a specific line. These positional sacrifices don't depend on exact calculation; they depend on intuition and experience and recognizing similar patterns we have seen before. See also the post Playing d5 Against the Gruenfeld.
However, it should be pointed out that the struggle for the initiative doesn't mean you must sacrifice something! Many times a strong POSITIONAL move is all it takes to gain the initiative.
In those cases where there is a possibility of attacking, how does one arrive at that conclusion? For example, the realization that there may be a good chance that a K-side attack will be successful is first based on general considerations. It's after conducting a general reconnaissance that one can get down to calculating variations.
Back in the old days gambits were played to attack the King and with the mindset that the thing to do was accept everything that was offered; defensive technique was often very feeble.
With the advent of modern positional play things changed. Defense improved and the goal was to seek active resistance and launch a counterattack. One result was that gambits became less common although at the time Steinitz was propounding his positional theories Mikhail Chigorin remained successful using gambits. In his match with Steinitz, Chigorin scored +14 -6 =5 using gambits. How did he do it? He did it, not by going all out for attacks on the King, but instead, he played for the initiative...long term pressure based on a lead in development, more space, control of the center.
Alekhine was also famous for his strategy, yes, strategy, of sacrificing a Pawn for the initiative. It's interesting to note that GM Alex Yermolinsky plays a lot of gambit lines because early in his career he tried pure positional chess, but it didn't give the results he wanted. He credited Soviet GM Mark Tsetlin with teaching him the “single most important thing” (Yermo's words) and that was the value of the initiative. Up until that time Yermolinsky had regarded the initiative a being a reward for correct positional play.
In fact, Yermolinsky wrote that the number of Pawns is just another positional factor that must be taken into account along with all other factors, adding that with a Pawn sacrifice you give a little and you get a little. He observed that when you sacrifice you must be patient and not expect immediate returns. When Alekhine annotated his game against Fahrni played in Mannheim, 1914 he wrote that if Fahrni, who had accepted a Pawn sacrifice, had played a different 8th move then white would have been able to gradually strengthen his position.
There is still a risk in playing a gambit, the opponent may find a successful defense and one's initiative peters out. But, as David Bronstein observed, you can't just strive for a game with no danger of loss. “You have to look for double-edged action. Given the present day level of technique, it is impossible to beat a strong opponent if you do not allow him any counter chances.” Bronstein went on to point out that in the process you do have to try and ensure that your chances are superior to your opponent's and the position suits your tastes and style.
In the following game Geller shows us what the initiative is and how to use it.