One does not lead the other, they trade independently of each other the reason that they trade with each other is arbitrage. An institution can come into the pit and start buying 1000's of contracts raising the price of the futures, this also drives the price of the ES higher. The S&P500 follows because stock baskets are being bought to lock in the momentary discrepancy in price or to hedge the liquidity providers in the pit. The ES follows the pit contract for the same reasons, if you can buy one at a discount to the other it is risk free money. The original function of a futures market was to hedge a commodity or cash position. Follow is the wrong word to use. They have a tight relationship based on fair value, any discrepancy is risk free money, less execution risk.