Camtasia showing cash and futures. Watch as the green lights up indicating an uptick on last trades for each stock. A numeric indicator shows net upticks vs downticks. When the numeric indicator jumps from +6 to +14 the ES ask gives way with some 1138 contracts "backing away". allowing the trading pair to move higher. <img src="http://www.elitetrader.com/vb/attachment.php?s=&postid=1236385">
Cash leads futures. There are exception of course. I can't imagine using SPX as disseminated. Build a synthetic SPX via a fungible synthetic basket. Personally, I refuse to build a synthetic SPX due to size and worthless NYSE quoting. I build a nas100 using NBBO/2 for midpoint, not last trade, fed into LMT-expo.
This shows what you are describing Riskarb regarding synthetic vs disseminated data. The larger bolder number above is a synthetic INDU quote and the smaller figure is NYSE disseminated INDU quote. Both quotes are from IB data source. You can readily see that the disseminated quote badly lags. Anyone using that data to calculate real time premiums or discounts to identify arbing programs is quite behind the 8 ball IMHO. <img src="http://www.elitetrader.com/vb/attachment.php?s=&postid=1236470">