good naked short analogy, but say it was your current g/f and her dad owned a liquor store where she was allowed to buy beer at wholesale prices on mondays only, when sales were down. She gets the beer for $9 a case. She then takes it to her apartment to stockpile it, knowing that frat boys have parties every weekend. She knows you have no money, but also knows you can get higher prices for the beer than she can retail it for. So you and her have an understanding. Every week you borrow her beer and sell it for $24 a case because demand is higher on those days. You catch up to her on Monday when the beer is cheaper and pay her back. This is a form of legal short-selling.