When I scratch my left nut -- S&P goes up. When I scratch my right nut -- S&P goes down. Glad to help.
Trader28Lite is right and UncleJ brings up an important point. I can't prove this, but I believe it to be true. A lot of plays on the index futures are motivated by arbitrage. There is a mathematical relationship between the futures price and the value of the index, but it is really difficult to exploit deviations from this mathematical relationship unless the gap is big because of transaction costs and the difficulty of executing 501 trades simultaneously (101 in the case of the NASDAQ 100). It does happen though. Often before the markets open there is sizable deviation between the price of the index futures contract and the fair value of the futures based on the value of the index. If the futures are overvalued relative to fair value, arbitrageurs will buy the index and sell the futures. If the futures are undervalued relative to fair value, arbitrageurs sell the index and buy the futures. When enough trading has taken place, the gap closes and they make their riskless profit. This is why when the futures are highly overvalued relative to fair value before the open, you will see the futures contract sell off right on or soon after the open. When it is highly undervalued before the open, you see a lot of buying on the contract on or soon after the open. In the cash market, the opposite takes place, which is why an overvalued index contract before market tends to imply a higher open for stocks. As for arbitrage between the big and mini contract, it's pretty much a no-brainer. The big contract is worth $250 times the value of the index, while the mini is $50 times the value of the index. the big contract trades in increments of 10 ticks per handle, while the mini trades in increments of 4 ticks per handle. Since the big contract can trade at 0, .10, .20, ..., .80, .90, while the mini can only trade at 0, .25, .50, .75, there's often a small arbitrage that can be executed. Suppose the big S&P contract is trading at 1450.20/1450.50 while the mini is trading at 1450.25/1450.50. Often the locals will try to get filled on the bid on the big contract at 1450.20 then immediately sell 5 times the number of contracts of the mini. Thus, they buy at 1450.20 and sell at 1450.25. Then all they need to do is cover the trades at a 0 or .50 and they make $12.50 for every big contract arbitraged. If you've ever listened to S&P500 Pit Squawk, you know that these trades happen. Arbitrage is the reason that the big and mini contracts move together.