In the preliminary election, voters selects their preferred candidate. If one candidate reaches the election threshold (usually fifty percent), they are declared elected. Otherwise, the top candidates (usually the top two) are placed on a secondary ballot. Whoever receives the most votes on the second ballot is declared elected.
30 voters. 50% threshold. Maximum of two candidates in the second round.
First ballot:
No candidates have achieved the threshold, so a second ballot is necessary. The top two candidates (Andrea and Brad) are placed on the second ballot.
Second ballot:
26 voters:
Andrea is elected.
In many voting systems, political parties hold primary elections before the general election. This is not the same as a runoff ballot. In a runoff ballot, all candidates are placed on the inital ballot, and all voters are allowed to participate in the vote.
This system encourages tactical voting in a manner very similar to plurality voting: voters are encouraged to avoid voting for candidates who do not have a chance to contend for the second ballot. However, this is a lower threshold than contending for the election.
If voters strongly dislike one candidate, they are also encouraged to vote for a candidate who they believe can defeat that candidate in the runoff.
See also: Instant runoff voting