They are making it convenient to long BTC and short Altcoin/BTC ratios if you care to get as much SegWit2x-coin as you can.
In the event of a hard fork that results in a chain split, Bitfinex is also faced with the challenge of resolving peer-to-peer financing issues. Because we have sufficient time to plan for this event we have elected to handle the accounting of peer-to-peer financed trading similar to how stock spinoffs, dividends, and distributions are handled in equity markets.
Specifically, in the case of a hard fork event, lenders will receive both BTC and B2X. Anyone that is short BTC/USD or long any BTC trading pair (ETH/BTC, LTC/BTC, etc.) will owe B2X to the lender, effectively making the user short B2X.
An exception is being made where BTC is borrowed but is not in use as financing collateral. In that case, B2X will accrue to the lender, [not the borrower]. Users that are financed long BTC/USD or short any BTC trading pair will receive B2X.
Bitfinex staff will be available on Whalepool Teamspeak and Bitcoin Mumble servers from time to time to answer customer questions. We will also be building a FAQ document to clarify edge cases and other nuances as needed.
In case you missed the news, you can now also trade chain-split tokens (CST) for SegWit2X - or at least follow their price action for price discovery information.