Clicking a link to one of the commercial partner sites takes you to a warning that you are leaving FamilySearch. When you agree, it takes you to the partner site. But it does not log you in, if you are not already logged in. If you are already logged-in, it conducts a search on the partner site using the data from the Family Tree Person page ... very handy!
If you are not already logged-in, you will still need to sign in using your username and password for the commercial site (or it will allow you to sign up for a trial account).