This may sound like an unfair system to those who are used to trading with confidence that their profits are settled in full, but it's a compromise that has been made in order to offer the high leverage. If you want to guarantee that your profits are not reduced by any socialised loss, then use CryptoFacilities or Coinpit. They use a "termination" procedure in the event of a margin call not being filled. This simply terminates the contract and sends the portfolio value of the losing counterparty to the winning counterparty. It's worth noting that the system has been incredibly robust, with very minimal socialised losses being triggered on OKCoin and none on BitMEX as of November 2015.
•   Liquidity Risk — In certain circumstances, such as the disruption of the orderly markets for the bitcoin futures contracts in which the Fund invests, the Fund might not be able to acquire or dispose of certain holdings quickly or at prices that represent true market value in the judgment of ProShare Advisors. Markets may be disrupted by a number of events, including but not limited to cyber-attacks, economic crises, natural disasters, new legislation, or regulatory changes inside or outside the U.S.

Currently the front month future is the January contract which at last check could be sold for $17,600.  This contract settles in a cash transfer based on the 4:00 pm eastern bitcoin auction price on Wednesday January 17, 2018 established by Gemini who is partnering with Cboe Global Markets.  The bitcoin price at Gemini is close to $16,600 so I will use that for the underlying bitcoin price in this example. 

The Custodian is responsible for safeguarding the Funds’ cash and securities, receiving and delivering securities, collecting the Funds’ interest and dividends, and performing certain administrative duties, all as directed by authorized persons. The Custodian is also responsible for the appointment and oversight of any sub-custodian banks and for providing reports regarding such sub-custodian banks and clearing agencies.


The exceptions to withholding for Capital Gain Dividends and short-term capital gain dividends do not apply to (A) distributions to an individual foreign shareholder who is present in the United States for a period or periods aggregating 183 days or more during the year of the distribution and (B) distributions attributable to gain that is treated as effectively connected with the conduct by the foreign shareholder of a trade or business within the United States under special rules regarding the disposition of U.S. real property interests as described below. The exception to withholding for interest-related dividends does not apply to distributions to a foreign shareholder (A) that has not provided a satisfactory statement that the beneficial owner is not a U.S. person, (B) to the extent that the dividend is attributable to certain interest on an obligation if the foreign shareholder is the issuer or is a 10% shareholder of the issuer, (C) that is within certain foreign countries that have inadequate information exchange with the United States, or (D) to the extent the dividend is attributable to interest paid by a person that is a related person of the foreign shareholder and the foreign shareholder is a controlled foreign corporation. If a Fund invests in a RIC that pays Capital Gain Dividends, short-term capital gain dividends or interest-related dividends to the Fund, such distributions retain their character as not subject to withholding if properly reported when paid by the Fund to foreign shareholders. A Fund is permitted to report such part of its dividends as interest-related and/or short-term capital gain dividends as are eligible, but is not required to do so.
  •   Theft, loss or destruction. Transacting on a blockchain depends in part specifically on the use of cryptographic keys that are required to access a user’s account (or “wallet”). The theft, loss or destruction of these keys impairs the value of ownership claims users have over the relevant assets being represented by the ledger (whether “smart contracts,” securities, currency or other digital assets). The theft, loss or destruction of private or public keys needed to transact on a blockchain could also adversely affect a blockchain company’s business or operations if it were dependent on the ledger.
Since you bought 68.4246 BTC, you want to short 68 BTC and lock in the USD value. You will have 0.4246 BTC left over, which will give you a slightly long bias. You can either fix this by only buying 68 BTC in step 1 or by giving yourself more short exposure by shorting 69 BTC. Remember that CryptoFacilities contracts are inverse (which allow locking in USD) and are denominated in BTC
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