What is the influence of the ES to the actual index in terms of order flow influence

What is the relationship between E-mini S&P futures and the actual S&P index? Besides the obvious, one is cash the other is a futures product. What I'm basically asking is how does buying/selling in one product actually influence the other? If for example someone sells 1000 contracts of E-mini futures.. how does that affect the S&P500 Index short term direction? The trade they've made should only affect the futures market. The seller isn’t technically selling shares in each individual stock within the SP500, so how does the cash market order flow adjust to this hyperthetical sell order within the futures market?