Our two-party system makes it difficult for third party candidates to gain traction. Given a choice between an unlikely third party win and a less desirable two-party candidate, some feel that a third party vote would be a waste of their vote.
Here are the rules of instant-runoff voting as described by Wikipedia: "Voters rank candidates in order of preference, and their ballots are counted as one vote for their first choice candidate. If a candidate secures a majority of votes cast, that candidate wins. Otherwise, the candidate with the fewest votes is eliminated. A new round of counting takes place, with each ballot counted as one vote for the advancing candidate who is ranked highest on that ballot." This continues until there is a winning candidate.