is that where you get to change your vote once you start seeing the returns?
IRV challenges the power of the two parties. It eliminates the “Don’t vote for the [non-Dem or Republican] candidate or you’re wasting your vote.” argument because you can vote for your preferred candidate first, then list the major party candidate. The major party candidate still gets your vote if your preferred candidate doesn’t win.
It does have its own problems though.
You have to understand the way IRV works to understand why they (correctly, in my view) think that it will effectively guarantee the election of more left-leaning, and lefter-leaning candidates. It's like this: let's say there are four candidates for a city council seat: Green, Democrat, Republican, Libertarian. You vote by "ranking" your choices, ie, let's say 1)Rep and 2) Libertarian. The first count of the vote goes down the list and if there is no candidate with more than 50% of the vote, the lowest vote getter is eliminated, and those people who voted for him have their second choice counted in a re-count. You keep going in similar fashion until a majority candidate is found.
So, here's the trick, if you're a "progressive": you don't have to campaign to your left to get the lefty votes. Let's say you're the 'Rat candidate in a slightly Rep-leaning district. You can campaign as a moderate, and pull some Rep votes (there are always some bonehead "i vote for the man not the party" types). It's a pretty safe bet that the Green voters are going to mark you down as No. 2, so you'll eventually get their votes. The same strategy won't work for non-RINO 'Reps and Libertarians, since their voters demand clear talk, principles, and character. In a district that can't reliably elect a conservative without a runoff, IRV will always favor the liberal/progressive candidate.