After selecting a timeseries and drawing a box around a movement of interest, by default change will be selected, and you should see a line segment showing you the change from start to end.
The system will take the change you've selected and find other periods with a move at least as extreme as the one you've chosen. Provided similar periods are found, the system will show you the typical forward path projection after that change occurred.
NOTE: for Rates timeseries, change is a level, not percentage change