Overview
The Volume Rate-of-Change ("ROC") is calculated identically to the Price ROC, except it displays the ROC of the security's volume, rather than of its closing price.

Interpretation
Almost every significant chart formation (e.g., tops, bottoms, breakouts, etc) is accompanied by a sharp increase in volume.  The Volume ROC shows the speed at which volume is changing.

Additional information on the interpretation of volume trends can be found in the discussions on Volume and on the Volume Oscillator.