If a package/ebuild fails to compile on gentoo you can use this feature to continue where it failed. Useful if a compile fails and you need to make manual edits to the files. Or if a compile fails because of out of memory, can happen when you compile chromium. Run the following command:

FEATURES=keepwork emerge -av <package>

If you don’t use the “keepwork” feature, gentoo will unpack the source code again, configure and restart the compile, which might not be ideal in all cases.