Calculations for the index starts tomorrow...all tracking funds bought today in the final half hour. Thats why volume went from a paltry 30M to over 70M in the last 30 minutes. Look at the charts, time and sales, etc.
Smart money uses a stop so they don't get raped. You better not be playing in the morning without stops. Make your money and sell then you don't worry about next week, next year just a little more tax a lot more money.