To use it select an object with motion clips on it. Click a motion clip or move the time sliter above or after a motion clip. The controls will then get the clip right under or before the time slider.

Use the mouse wheel on the dial to increase or reduce the length of the clip. You can also extend the length to the next clip. Or to the last added key in the clip. Note that even if you reduce the length of the clip you wont lose the motion from the cut away part. You can just size it and you get it back.