Friday was one of the four times a year when the S&P 500 rebalances itself. This means that some stocks leave this index and some come in. This also means that all the indixes and exchange-traded funds that track the S&P 500 (and there's many) have to also rebalance themselves to reflect the changes. S&P Dow Jones Indices, the parent company of the indicator, estimated this rebalancing would prompt $32 billion in required index trades.
Click the audio player above to hear the full story.