Chris and Trish Meyer posted another part in their series CMG Hidden Gems: Chapter 18 – Nesting. The focus here is on how to efficiently navigate and edit a chain of nested comps, and looking under the hood a bit to help you troubleshoot. Especially interesting is coverage of ETLAT or "Edit This, Look at That," which is the result of locking a Comp Viewer while working in another comp's Timeline and with the Effect Controls panel. ETLAT lets you preview changes downstream while working in a nested comp.
AE Help explains:
"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."
CS5 added several more several more features so that now "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."