It is an index that looks like the final evolution by weighting the Bollinger band with exponential smoothing and volume. The base Line is my EvMA as volume weighted EMA, so it is quite responsive. The standard deviation is also exponentially smoothed, and the reaction is too good to handle, so it is further smoothed by EMA. Charts without volume are not weighted with volume as 1. It seems that the usage in trading is the same as the Bollinger band