But many were unable to sell enough futures, or even stocks, during the panicky days of October.

Many traders who sold futures may not be able to meet delivery obligations under the contracts.

Traders simultaneously bought oil and sold futures forward.

Many institutions were thus unable to sell futures; they had to sell actual stock, at huge losses.

Selling futures, of course, pushes down their price.

Portfolio insurance involves selling options or futures against the underlying stock that is carried in a portfolio.

Therefore, they continued to sell futures throughout the morning and early afternoon at tremendous discounts to the prices in the stock market.

In 1987, the primary form of portfolio insurance involved institutions selling stock-index futures as prices declined.

And it will then plan to limit its risk by selling stocks or futures if the value of that portfolio does fall.

For instance, an investor might buy the issues that make up a stock index and then sell futures on the index.