It passes the Rate of Change data through a Butterworth filter which creates a smooth line that can allow for easier detection of slope changes in the data over various periods of times.
The butterworth filter line and the rate of change are plotted together by default. The values for the lengths, for both the butterworth filter and the raw ROC data, can be changed from the format menu (through a toggle). The shorter the Butterworth length, the closer the line is fitted to the raw ROC data, however you trade of with more frequent slope changes. The longer the Butterworth length, the smoother the line and less frequent the slope changes, but the Butterworth line is farther of center from the raw ROC data.