From Hendrik Hertzberg a primer:
Instant runoff voting, as you (being a reader of this blog) probably know, eliminates the “spoiler” effect in the one-winner elections standard in the United States; guarantees that the winner has at least grudging support, of a majority; and guarantees that a candidate whom most voters really, truly don’t want can’t get elected. The way it works is this: (a) You list your choices in order of preference. (b) If someone gets an outright majority of first choices, that’s it. (c) Otherwise, there’s an “instant runoff”—the biggest loser gets dropped from the counting and his or her voters’ second choices get counted along with everybody else’s first choices. (d) Repeat (c) till someone has a majority, though this is almost never necessary. I.R.V. is used in Australia and Ireland, where voters like it fine, and in several American cities—including Burlington, Vermont.