With only two candidates running in a second round, runoffs supposedly ensure that winners have majority support. But large drops in turnout are common. Runoff elections also are costly. Second elections would cost taxpayers millions of dollars in a big state like Pennsylvania, and candidates would have to slug it out for several more weeks and duplicate expensive efforts to get voters to the polls. Modern technology allows a far better way to seek majority winners in elections that maximize voter turnout.
Voters have one vote but can rank candidates in order of preference.
The ballot count operates as a series of runoffs. If no candidate wins a majority of first choices, then the last-place candidate is eliminated. Ballots of that candidate's supporters are reallocated to the next-choice candidate on each ballot. That continues until one candidate remains. Voters can choose to rank only one candidate, but they have every incentive to rank more because ballots only move to a lower choice once a higher choice is eliminated.