it's not a spin off though. nothing is leaving btc. no value is going away. someone else just created something out of thin air. also, bch was never a part of btc. it's simply something new that is using the past history of btc as a starting point, which creates new coins out of thin air, and distributes them to all the holders of btc.
No part of this makes sense for a short to have to go and buy bch.