Not the prints on ecns necessarily but the ecn activity. How quickly they start bidding up or down when the NYSE up or down ticks. These buy or sell programs on NYSE issues tend to show their hand if you force them. Like sometimes, when the spread is more than a penny (say 5 cents), I'll keep bidding to tighten the spread, hoping to get a buy program to follow me. If he does, I generally know that there is a buyer at that level. Testing the waters like this has helped me get a better read on situations.
Quote from chud:
Steve, do you watch all the prints that go off on the ECN's? I've been filtering those out and only watching NYSE prints.