The New York Stock Exchange and NYSE Amex Cash Markets on Wednesday invoked a rule to smooth trading at the market open as futures tumbled following a spike in Italian bond yields.

Invoking rule 48 allows the exchange to suspend price indications that help determine the floor price at the open during regular sessions. Bypassing the requirement helps speed the beginning of trading.

S&P 500 futures fell 28.2 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration of the contract. Dow Jones industrial average futures lost 198 points and Nasdaq 100 futures dropped 39 points.

Among the triggers for invoking the rule are substantial activity in the futures market before the open, according to the exchange's website.

(Editing by Padraic Cassidy)