From the release notes for 1.6.2
@{-1}is a way to refer to the last branch you were on. This is
accepted not only where an object name is expected, but anywhere a
branch name is expected and acts as if you typed the branch name.
E.g.git branch --track mybranch @{-1},git merge @{-1}, and
git rev-parse --symbolic-full-name @{-1}would work as expected.
and
git checkout -is a shorthand forgit checkout @{-1}.