Those 'circuit breaker' levels are designed to reduce market volatility by forcing a pause in trading activity for certain periods of time. The NYSE on its Web site explaining the rules following what it said were questions about how circuit-breakers work. Circuit breakers to stem futures trading kicked in before the markets opened in New York on Friday after markets elsewhere in the world fell sharply.

The NYSE ended the day off by more than 300 points; The technology-laden NASDAQ shed 51 points. Neither was anywhere near as large as .

Basically, circuit breakers are used to cut off automated trading program connections between large brokerage houses and the computers on the NYSE trading floor for varying lengths of time, depending in the severity of the decline and the time of the day it happens, according to the exchange's

Larger brokerage houses "have their computers directly connected to the trading floor on the stock exchanges, and hence can program their computers to place direct huge buy/sell orders that are executed in a blink," the FAQ said. "This automated connection allows them to short-cut the individual investors who must go thru the brokers and the specialists on the stock exchange."

Because of the -- in microseconds -- circuit breakers are aimed at slowing a market crash.