The SpotGamma Implied Volatility Index forecasts what range we expect on the day.  As the market moves up or down, the amount of Gamma shifts, and reflexively, the amount of volatility changes.  The percent on the y-axis shows the expected move, with the higher the green line in the chart, the higher the expected volatility, plotted against the range of strike prices for the option.

Specifically, the SIV curve specifically reflects how much volatility will take place relative to a shift in the underlying price.  If you are an options trader, you can look to see how volatility will increase or decrease as we move higher or lower, and then structure positions. You can look at the change in this model over time, by clicking on different dates, and when you see an imbalance in call Gamma or put Gamma, you should expect more movement over the coming day.

SpotGamma Subscribers can access this chart for the SPX, SPY, NDX, QQQ, RUT and IWM.