Because volume correlates highly with volatility, this ratio gives a nice real-time updating of how much movement we can expect in the stock index. Note that the couple of times we bounced above 1.0 during the morning, relative volume tailed right back off. It's when we see persistence of high ratio readings that we generally see range extension and short-term momentum. In the low volume environment, we're more likely to see moves reverse before possibly continuing in their initial direction.