Edit this, look at that (ETLAT) and locked Composition viewers

Edit this, look at that (ETLAT) and locked Composition viewers

If a Composition viewer is locked, the Timeline panel for another composition is active, and the Composition viewer for the active composition is
not shown, then most commands that affect views and previews operate on the composition for which the viewer is shown. For example, pressing
the spacebar can start a standard preview for the composition visible in a locked Composition viewer rather than the composition associated with
the active Timeline panel.
This behavior facilitates a working setup sometimes referred to as edit-this-look-at-that (ETLAT). The most common scenario in which this
behavior is useful is the scenario in which you make a change in the Timeline panel for a nested (upstream) composition and want to preview the
result of the change in a containing (downstream) composition.
Note: ETLAT behavior works for keyboard shortcuts for zooming, fitting, previewing, taking and viewing snapshots, showing channels, showing
and hiding grids and guides, and showing the current frame on a video preview device.
To prevent this behavior, unlock the Composition viewer or show the Composition viewer for the composition that you want to view or preview.
See this video on the Video2Brain website to learn about the improvements in ETLAT (edit-this-look-at-that) workflow in After Effects CS5.5 and
later.