"It takes time to execute a program trade," said one trader who asked not to be identified.

The total net decline to the Dow from program trades was 30.44 points.

In practice this means that all program trades are entered with the aid of a computer.

But program trades could still be executed by brokers on the floor of the exchange.

The second session is designed to handle certain forms of program trades now done in London.

Consequently, if someone does have a big program trade one way or another, you could see more price movement than would normally be expected.

But brokers on the floor could still execute program trades.

After the delays in the automatic system, any program trades that had been entered into it during the period would be released.

As a result of the those rules, there were far fewer program trades than normal yesterday.

Analysts said they had to go back to late February to find a program trade with such a large basket of securities.