Unlocking a file changes how it is stored in the git repository (from a symlink to a pointer file), so this command will make a change that you can commit.
The content of an unlocked file is still stored in git-annex, not git, and when you commit modifications to the file, the modifications will also be stored in git-annex, with only the pointer file stored in git.
If you use git add to add a file to the annex, it will be added in unlocked form from the beginning. This allows workflows where a file starts out unlocked, is modified as necessary, and is locked once it reaches its final version.
Normally, unlocking a file requires a copy to be made of its content, so that its original content is preserved, while the copy can be modified. To use less space, annex.thin can be set to true; this makes a hard link to the content be made instead of a copy. (Only when supported by the file system.) While this can save considerable disk space, any modification made to a file will cause the old version of the file to be lost from the local repository. So, enable annex.thin with care.
# git annex unlock photo.jpg # gimp photo.jpg # git annex add photo.jpg # git annex lock photo.jpg # git commit -m "redeye removal"