'In' and 'Out' Points can be set on the timeline, allowing you to view a certain part of the timeline only.
Only one set of points can be applied at a time. The timeline outside the points will be greyed out.
If the points are set, and Looping is on, the media will only loop between the points.
The points are set from the 'Navigate' menu or the keyboard and can be set on the fly.
Keyboard shortcuts: I = In point; O = Out point; Apple / Ctrl T = Toggle points on/off; C = Clear points