In the case of major panic selling, the markets will be shut down with circuit breakers

NEW YORK (CNN) -- Monday's historic drop in the financial markets sent shivers through the investment community, but it wasn't enough to force a halt in share trading. In fact, it takes a considerably bigger plunge to trigger the stock market's so-called "circuit breakers"

The idea of closing the stock market temporarily for a cool-off phase came in the wake of the 1987 market crash, when the bottom fell out, in part because of panic-selling.

Under new rules that took effect at the start of July, the Dow Jones market index has to fall about 10 percent (which would have been 800 points Monday) before circuit breakers will stop trading for up to an hour.

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Should the market drop by about 20 percent (1600 points), the market is shut for up two hours.

And in case of a 30 percent drop (2400), trading is suspended for a full day.

Some critics say that circuit breakers make people bail out even faster than they would otherwise, thus worsening panic-driven selling.

Others point out that no one really knows for sure how the temporary suspension would work. The market has lost 10 percent of its value in one day just twice so far: in 1929 and 1987, well before circuit breakers existed.