Say we have the following voter preferences:
% of voters Their ranking
35% conservative > centrist > liberal
33% liberal > centrist > conservative
22% centrist > liberal > conservative
10% centrist > conservative > liberal
[E.g. the first row says that 35% of the voters prefer the conservative, over the centrist, over the liberal.]
Now whom should we elect? Which candidate best represents the will of these voters? Note that the centrist is preferred to the conservative by a huge 65% majority (the 2nd and 3rd rows). And the centrist is preferred to the liberal by an even bigger 67% majority (the 1st and 3rd rows). It seems clear that the centrist is by far the most broadly appealing candidate.
If we use Instant Runoff Voting, the centrist is eliminated first, and then the liberal defeats the conservative, 55% to 45%. With an ordinary delayed runoff, the conservative and liberal would be pitted head-to-head in a second election, where we would expect the same result (unless a significant number of voters change their minds between the first and second election).
But either way, the centrist doesn’t win. This is called the “center squeeze” effect, because the broadly appealing centrist is squeezed from both sides by candidates who absorb most of the support of the two main sides of the political spectrum. The diagram below illustrates the effect.