Quote from piezoe:
SPX responds to its components. The components may move before SPX and vice versa. When you see a major component, an example might be XOM, make a big move over say a week's time but SPX moves relatively little, or perhaps a little in the opposite direction, a divergence is created. You can reasonably expect in a case like that SPX will catch up with XOM and soon move in the same direction. Similarly, if SPX made a move not reflected in XOM, you might reasonably expect XOM to follow. If you plot XOM and a few other major components of SPX along with SPX on the same chart, the results could be enlightening, and might tip you off in advance to moves in the SPX components or in SPX, depending on which components are leading or lagging the SPX. Most of the time, of course, SPX moves in consort with its major components. But there are enough total components that occasionally one of the major components can lead or lag. That may create a trading opportunity.
Although you can't trade SPX directly you could trade an ETF like SPY, or a future like ES that closely mirror SPX.