In South Korea’s election this week, the conservative candidate Yoon Suk-yeol, of the People Power Party, defeated progressive Lee Jae-myung by just 0.73%.
Less reported in the international press is the fact that an alternative left-wing candidate, Sim Sang-jung, also ran. She received 2.33% of the vote, which almost certainly would have gone to Lee, of the ruling Democratic Party, had she not run. Sim "threw" the election to Yoon, as her votes would have given Lee a clear victory margin.
South Korean presidential elections reward the candidate with the most votes. This plurality requirement strongly encourages all voters on the right and left to converge around one unity candidate for each side. Small-party candidates can pull away votes from an ideologically similar, large-party candidate who might otherwise win.