How does Homebrew no longer need ownership of /usr/local? Today I ran brew update and after migrating the repository, it reported that it no longer needed ownership of /usr/local:

$ brew update
Updated Homebrew from 5371359 to 13f08a2.
...
==&gt; Migrating HOMEBREW_REPOSITORY (please wait)...
==&gt; Migrated HOMEBREW_REPOSITORY to /usr/local/Homebrew!
Homebrew no longer needs to have ownership of /usr/local. If you wish you can
return /usr/local to its default ownership with:
  sudo chown root:wheel /usr/local


This change seems a little dubious. How does brew accomplish this new behavior, apparently bypassing security controls?
According to this ruby script: update-report.rb, they've just moved stuff to /usr/local/Homebrew, which is still owned by you.