[Tasuki]

Bert,
The relationship between the cash and the futures is a fairly complex one. The "cash" simply means the $SPX index. Big hedge funds and Prop firms are able to buy and sell each and every stock in S&P 500 with huge computers and huge accounts, so in this way they can actually trade the index, or rather, the stocks within the index. Arbitrage traders look for imbalances between the cash index and the futures (full contract or emini) and they will buy the cheaper and sell the dearer. In this way, the cash and futures are always dancing close to each other and taking turns as to which one leads.

A further level of complexity involves the locals in the S&P full contract pit, who can and do buy and sell the eminis to square their positions when they get caught long or short.