In Czech Republic there will be parliament elections soon.
There are two major coalitions which have similar chance of winning (lets call them A and B). I prefer the B wins, even though I don't align with them 100%, but still I see them as better choice than A.
Who I really align with is a small party (C), which realistically does not have a chance of getting into the parliament.
In this is situation what makes more sense, if my main goal is to prevent the A from winning? Vote for B, so they are in better position, and can beat the A. Or cast my vote for C, because I align with them best, but the vote will be practically wasted, as they don't really have a chance.
EDIT: In Czech republic following electoral system is used: 200 members of the Chamber of Deputies are elected from 14 multi-member constituencies by open list proportional representation with an electoral threshold of 5%. Seats are allocated using the D'Hondt method. Voters can give preference votes to up to four candidates on a list. Candidates who receive preferential votes from more than 5% of voters are moved to the top of their list; in cases where more than one candidate receives over 5% of the preferential votes, they are ranked in order of votes received. source