Let's notice the relative weakness of the S&P 500 Depository Receipts (SPY) in relation to the PowerShares QQQ Trust (QQQ), exhibited by the juxtapostions of their trading moving averages.
The QQQ has found support at its fastest, upper-most moving average, while the SPY has violated its most "bullish" moving averages, and has spent the last few hours probing its middle postion moving average. The fact that big technology names such as Apple (AAPL), Amazon (AMZN), Intel (INTC), Cisco (CSCO), NetFlix (NFLX) and others are positive today in contrast to the banks and the industrials, which are heavy, accounts for difference.
That said, which index has the uppper hand? My sense from the chart work on AAPL and AMZN is that they are tired and peaking, which if accurate, means that the QQQ (NDX) will fall into line with the SPY (SPX) in the upcoming hours.