You don’t rank them last, you don’t rank them at all. Essentially that is a vote against them because your vote will go to someone else no matter what. The best part is it allows for third party candidates. You don’t like your party candidate? No problem, you can vote for a different person first and if that person doesn’t end up getting it, you might get your second or third choice but they would still be much better than getting the person you felt you should use your vote to vote against.