Just off the top of my head, I would say this may indicate that volatility is increasing on the NYSE and/or is waning on the NAZ. I think this can be cross verified with the TICK numbers on each specific index to help in drawing conclusions.
As to the usefullness, I'm not sure, but maybe it could be used to help determine asset allocation shifts between NYSE and NAZ stocks.
I will have to think about this one some more, but thanks for giving me something to think about.