When a professional equity options trader is forced to buy back a
short stock position (called a Buy-In) that hedges a long
delta options position and the stock opens higher the next
day.
You go out
short 10000 shares of EET against a long 10000
delta options position. Overnight you are forced to buy back your shares at $40. The next
day the stock opens at $42. You have just made $20000, hence the term Buy-Win.