How `git checkout -` works on low level

How can I get a previous branch/commit using Git plumbing commands or status files?

Edit: The question is - how can I get this information (the previous branch or commit) without performing an actual checkout? This is needed for a tool which works on top of Git, not for a regular Git scenario.

By taking a look in the code:

if (!strcmp(arg, "-"))
    arg = "@{-1}";

The way to access the previously checked out branches is documented in revisions doc on @{-n}

Now, to resolve the branch name behind @{-n}, the solution is git check-ref-format --branch:

With the --branch option, it expands the “previous branch syntax” @{-n}. For example, @{-1} is a way to refer the last branch you were on. This option should be used by porcelains to accept this syntax anywhere a branch name is expected, so they can act as if you typed the branch name.

$ git check-ref-format --branch @{-1}
my_branch
$ git check-ref-format --branch @{-2}
master

There is also the git rev-parse --symbolic-full-name solution:

$ git rev-parse --symbolic-full-name @{-1}
refs/heads/my_branch
$ git rev-parse --symbolic-full-name @{-2}
refs/heads/master