Q: What caused the flash crash that sent the market reeling on Thursday, May 6, and how can I protect myself?

A:  Investors were horrified when during a brief 15 minute period on May 6, stocks seemed to be in freefall.

The damage is explained in detail in USA TODAY’s coverage of the historic event:

Details of what precisely caused the horrifying crash will be emerging for months. But at the core of the problem has to do with the way investors’ orders are filled by the largely electronic exchanges.

Take Procter & Gamble (PG) stock for instance. The stock is listed with the New York Stock Exchange. which is the primary market for that stock to trade. But P&G’s stock actually trades on many other markets, including the Nasdaq. BATS and Direct Edge.

This is a key fact to remember: Stocks don’t just trade on their primary exchanges.

On May 6, unrest in Greece helped cause a large spike in stock selling. When an unusually large amount of sell orders for NYSE-listed stocks like P&G hit the market, the NYSE went into slow mode. That meant the sell orders were diverted to other markets, including the Nasdaq, which doesn’t have as many buy orders for the stock.

This was a problem because the other markets, outside the NYSE, didn’t have as deep of a book or orders from buyers. The price of stocks plunged in order to meet the extraordinarily low bids on the other exchanges.

These flash crashes are especially infectious since so much of our stock markets are electronic. Some systems are programmed to not buy stock if the stock is falling. That means no buy orders were emerging to satisfy the growing number of sell orders.

Meanwhile, there were human aspects that only made the situation worse. Some traders panicked when they saw the NYSE go into slow mode on some stocks, worrying that there might be a huge wave of selling coming.

Regulators will need to take a hard look at what happened with the market, and ensure that the malfunction doesn’t occur again.

But there are things you, the investor, can do to protect yourself in the meantime:

Don’t get overly wrapped up in hour-by-hour moves of the stock market. Had you just hung on to your shares, the whole mess sorted itself out by the end of the day. If you didn’t sell, the event was little more than a very strange day.

Be careful of so-called stop market orders. These are orders you place with your broker to automatically sell stocks if their prices fall to a certain point. A stop market order will trigger and sell your stock at any price, even if the stock is crashing. If you use a stop limit order instead, you can instruct your broker to not sell your stock in a firesale.

Matt Krantz is a financial markets reporter at USA TODAY and author of Investing Online for Dummies and Fundamental Analysis for Dummies. He answers a different reader question every weekday in his Ask Matt column at money.usatoday.com. To submit a question, e-mail Matt at mkrantz@usatoday.com. Click here to see previous Ask Matt columns. Follow Matt on Twitter at: twitter.com/mattkrantz