You are required to post cash collateral in order to take out a futures position. It's calibrated so that the cost of abandoning that collateral is large enough

You are required to post cash collateral in order to take out a futures position. It's calibrated so that the cost of abandoning that collateral is large enough to basically be a gun to your head. Furthermore, when the price of the contract moves, they mark-to-market your collateral and credit/debit you your daily gains or losses. Basically, the people holding the contracts have to either sell at a negative price to cover, pay to take delivery, or give up cash collateral that's worth even more money than the previous choices.