Following the stock market crash of 1987, the New York Stock Exchange instituted circuit breakers to halt trading during steep market declines. One such circuit breaker, called a "side-car" was instituted during the first five minutes of trading.
At the open, during the first five minutes, NYSE officials would put all program trades (defined as orders involving baskest of 15 stock or more and dollar values of $1 million or higher, on hold, as if stuck in a side-car. The rule would be instituted if the CME's S&P 500 lead-month futures contract declines 12 points from the previous day's close. The side-car rule did not apply to the last 35 minutes of trading.
When triggered, all program trading market orders entered in SuperDot for NYSE-listed component stocks of the S&P 500 were diverted to a separate blind file for five minutes. After the five-minute period, buy and sell orders were paired off and become eligible for execution. If orderly trading in a stock could not resume, trading in that stock was halted and imbalance information was publicly disseminated.