As futures contracts, the e-minis represent an agreement to buy or sell the cash value of the underlying index at a specified future date.
The e-mini S&P 500, for example, has a contract size of $50 times the e-mini S&P 500 futures price. If the value of the e-mini S&P 500 is $1,320, the value of the contract is $66,000 ($50 x $1320)
E-mini: A position with five e-mini S&P 500 futures contracts (that each trade at one-fifth the size of the full-sized contract) has the same financial value as one full-sized contract in the same contract month (assuming both positions are on the same side of the market).
Long and short & Volatility
As long as there is volatility (changes in price), traders can theoretically make a profit.