Explainer: How a GameStop share pullback could hurt some investors

Shares of GameStop, which had seen a spectacular rally, fell 31% on Monday to close at $225, 53% lower than their Jan 28 peak of $483.

Analysts worry some new or inexperienced investors could face losses if they bought in as the stock was peaking.

Particularly vulnerable would be those who bought "on margin," using money borrowed from brokers to buy extra shares.

The strategy can boost gains when share prices are on the way up, but magnify losses on the way down especially if brokers issue "margin calls" requiring clients to add money or face forced sales to bring an account's equity back to required levels.

Brokers are not required to notify clients when they sell shares in a margin call, "although most do so as a courtesy," according to the Financial Industry Regulatory Authority https://www.finra.org/investors/insights/margin-calls, the industry's self-policing body.

Riley Adams, a 31-year-old financial analyst who runs youngandtheinvested.com, a financial blog aimed at millennials, said if investors bought on margin late to the party "you're definitely exposed right now."

Thomas Peterffy, chairman of Interactive Brokers, estimated about half the platform's 1.2 million accounts are margin accounts. Thousands of margin calls occur on a typical day and the rate increased last week, he said. Peterffy said about 27,000 accounts had some sort of position in GameStock, and many positions liquidated were owned by traders who had shorted GameStock.

Representatives for brokerages TD Ameritrade and Robinhood declined to share details about how many clients may have traded GameStop shares on margin. A Schwab representative did not respond to questions.

However, Robinhood has restricted https://robinhood.com/us/en/support/articles/changes-due-to-recent-market-volatility buying shares, which limits investor exposure on its platform.

Options bets on GameStop shares helped fuel the stock's breathtaking rally. Investors are gauging to what extent they could exacerbate a decline.

Some market watchers say a "gamma squeeze" - where market makers who have sold large numbers of calls to investors balance out their positions by buying the underlying stock - was a key driver of GameStop's sharp rally.

In theory, a sharp drop in the stock could prompt those same market makers to unload their shares, potentially speeding up a plunge. That's what happened last September, after investors piled into call options on tech-related companies such as Amazon.com Inc and Alphabet Inc. The unwind of those positions helped fuel a sharp decline in the Nasdaq.

For now, however, it appears that open interest in GameStop calls has not accumulated significantly, as many buyers of those contracts have traded them the same day, according to Christopher Murphy, co-head of derivatives strategy at Susquehanna Financial Group.

More contracts have remained open among GameStop puts, according to data from Trade Alert. Another key risk would be to market participants who sold those puts, wagering that GameStop shares would not fall below a certain level, if the stock were to tumble significantly below their targets.

But because options prices already account for gigantic swings in the stock, "people who are short options might not lose as much as you think," Murphy said. Overall, GameStop options are now pricing in daily moves of about 27% over the next month, versus around 8.5% at the start of January, according to Trade Alert.

On the other hand, investors who have held onto calls to position for further gains in GameStop shares could lose their entire investment should the stock not hit their targeted price before the options expire.

A slide in GameStop shares would see short sellers looking to capitalize by finding "attractive exit points," said Ihor Dusaniwsky, managing director of predictive analytics at S3 Partners. Short sellers sell borrowed shares in a bet that prices will fall and the shares can be bought back at a lower level.

At the same time, a slide could draw new shorts looking to jump on any downward price momentum, Dusaniwsky said. The number of GameStop shares shorted has fallen by more than half in the past week, as shorts have covered their bets, Dusaniwsky said.