So are you saying that someone with $100K in equity can carry $820K in open posiition exposure overnight without a problem just because half of it is long and half of it is short??
They're not really offsetting exposures. Suppose they've got 10,000 shares of Stock A @ 40 long and 10,000 shares of Stock B @ 42 short. The exposure isn't only $20,000 (or -20% of equity as you noted).
Suppose both stocks come out overnight with news and the long stock (Stock A) opens the next day @ 30 (i.e., down 10) and the short stop (Stock B) opens @ 52 (i.e., up 10). The trader is now suddenly down $200,000 (twice their equity)
They're not really offsetting exposures. Suppose they've got 10,000 shares of Stock A @ 40 long and 10,000 shares of Stock B @ 42 short. The exposure isn't only $20,000 (or -20% of equity as you noted).
Suppose both stocks come out overnight with news and the long stock (Stock A) opens the next day @ 30 (i.e., down 10) and the short stop (Stock B) opens @ 52 (i.e., up 10). The trader is now suddenly down $200,000 (twice their equity)
