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In his job as a financial educator, Keith had spent a fair amount of time breaking down the act — and sometimes art — of short selling, in a way that less savvy customers could understand. When a trader believed a company was in trouble, and its stock was overvalued, they could 'borrow' shares, sell them, and then when the stock went down as they'd predicted, rebuy the shares at a lower price, return them to whoever they'd borrowed them from, and pocket the difference. If GameStop was trading at 5, you could borrow 100 shares, sell them for $500; when the stock hit 1, you bought back the 100 shares for $100, returned them, pocketing $400 for yourself. You paid a little fee to the lender for their trouble and came out with a tidy profit.
But what happened if the stock went up instead of down? What happened if GameStop figured out how to capitalize on its millions of nostalgic customers, who spent billions on video games every year? What if the stock went to 10 instead of 1?
What happened was, the short seller was royally screwed. He'd borrowed those 100 shares and sold them at 5. Now the stock was at 10, but he still needed to return his 100 shares. Buying them on the market at 10 meant spending $1000. And what was worse, when he'd borrowed the shares, he'd agreed on a timeline to return them. There was a ticking clock hanging over his head, so he had a choice — buy the shares back at 10 now, losing $500 on the deal — or wait a little longer, hoping the stock went back down before his time limit was up.
And what if he waited, and the stock kept going up? Sooner or later, he had to buy those shares back. Even if the stock went to 15, 20 — he was on the hook for those 100 shares. Theoretically, there was no limit to how much he could lose.
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Ben Mezrich (The Antisocial Network: The GameStop Short Squeeze and the Ragtag Group of Amateur Traders That Brought Wall Street to Its Knees)