Of the 300 members of their National Assembly, South Korea elects 253 members using first-past-the-post in single member districts and 47 members proportionally. The proportional component was recently amended, a short time before the 2020 election so that 30 members were elected in a compensatory manner: A party with 33% of the vote and zero district seats would elect more list members, and one with some district seats would win less, to bring their seat proportion closer to their vote proportions. A party with 33% of the vote and >100 district seats would win no list seats in the compensatory section. This is known as mixed member proportional (MMP), or the additional member system (AMS), whereas the system with list seats being awarded independently of district seats is known as mixed member majoritarian (MMM) or parallel voting, and is used for the other 17 PR seats.
Of course, this means that if a party wins a significant number of district seats, it is in their best interests to split into a party that wins zero district seats and much of the list vote and another to win district seats without winning list votes (which would not help an overrepresented party). This requires parties to ensure that most of their list vote goes towards one party, without any of the district vote leaking the same way, which could lose them district seats due to the spoiler effect. It's also only useful when the party actually wins district seats, which smaller parties generally don't.
Instances of this kind of split ticket voting is fairly rare in part because the MMP system is fairly rare, with the only users being Germany, Bolivia, Lesotho, New Zealand and various devolved parliaments in the UK (including Scotland). In comparison, over 20 use MMM and the number that simply use party lists is somewhere around 70. Neither of those systems give an advantage for that kind of tactical voting. Of course, the rarity of the system might be partially because the countries that do experience tactical voting end up eventually switching to a different system. Nonetheless, there have been accusations of "gaming" in Lesotho, Albania, Venezuela and also in the most recent Scottish elections, where pro-independence party Alba plans to run only list candidates. Of course, as explained linked tweet thread, it's entirely possible that votes for this new party hurts the pro-independence camp more than it helps.