In Karma unit test how does $timeout get injected?

I am not exactly sure how to ask this so I'll try to do it through code.

This is part of a unit test I wrote.

    var loginAfterCredentialsFailed = function($timeout) {
        // bunch of code
    }

    it('goes to login page because cached credentials were not found',
        inject(loginAfterCredentialsNotFound));

What I don't understand is how $timeout is resolved? Does the inject give you access to $timeout? What else does inject provide?

Thanks in advance.