Summary: Expand or hide a comp's content without having to exit the existing comp.
Description: Hold down Opt and the hide/expand arrows appear on all comps. Left click on a comp with Opt held down hides/expands the comp.
*This feature can only work if the comp being expanded is in exactly the same time as the original comp. If the comp being "peaked at" is out of sync with the original comp a message appears whenever the user attempts to edit these layers.