Inverse bitcoin contracts are usually denominated in USD terms. So if there's a contract value of $100 then each side -- LONG and SHORT -- puts down some portion of the contract value in margin. The contract is an agreement between traders to pay the other side an amount of bitcoin profit/loss as the price changes. Each contract has an expiration date, and some exchanges have periodic (daily or weekly) dates where the contract period's profits are "settled" even if the contract has not expired.
In addition, the securities of some foreign governments, companies and markets are less liquid, and may be more volatile, than comparable securities of domestic governments, companies and markets. Some foreign investments may be subject to brokerage commissions and fees that are higher than those applicable to U.S. investments. A Fund also may be affected by different settlement practices or delayed settlements in some foreign markets. Moreover, some foreign jurisdictions regulate and limit U.S. investments in the securities of certain issuers.
To seek its investment objective, as a cash reserve, for liquidity purposes, or as “cover” for positions it has taken, each Fund may invest all or part of its assets in cash or cash equivalents, which include, but are not limited to, short-term money market instruments, U.S. government securities, floating and variable rate notes, commercial paper, certificates of deposit, time deposits, bankers’ acceptances or repurchase agreements and other short-term liquid instruments secured by U.S. government securities. Each Fund may invest in money market instruments issued by foreign and domestic governments, financial institutions, corporations and other entities in the U.S. or in any foreign country. Each Fund may also invest in pooled investment vehicles that invest in, and themselves qualify as, money market instruments.
Elsewhere, here is the story of block.one, which "has raised about $700 million and counting" by selling EOS tokens that it says "do not have any rights, uses, purpose, attributes, functionalities or features." Block.one is using the money to build "a new blockchain architecture designed to enable vertical and horizontal scaling of decentralized applications," as its white paper explains, and the white paper also includes a disclaimer in bold capitals:
Making money arbitraging bitcoin futures can be extremely simple. Futures contracts typically trade at a premium, and all you have to do, starting with USD, is buy bitcoin at Spot price and sell futures of the same amount at premium price. Then just wait until expiration to make your arb profit in bitcoin (which you can then put in USD). Whether it's a weekly, monthly, quarterly, or any futures contract, as long as it's in a premium, you lock in the sale price and earn the arbitrage profit.