Let's say you're trying to figure out how to vote in the Wisconsin primary tomorrow, and either you don't have the option of voting for someone you actually support (because you support no one) or you want to use your vote strategically in the way that you believe is most effective.
It's an open primary, so you can vote for any of the 5 remaining candidates. You could simply pick the least objectionable person in the 5, perhaps John Kasich. But you've seen the analysis that says to vote for Kasich is to help Trump, because — based on polling — only Cruz has a realistic shot at beating Trump in Wisconsin. By helping Cruz get Wisconsin delegates, you advance the cause of keeping Trump from closing in on the 1237 delegates that will get him the nomination on the first ballot at the convention.
And you don't want Trump to get the nomination. You'd like to see the convention get to the second ballot, with the delegates released to begin voting for other candidates. But you're picturing Cruz at that point arguing strenuously that he's the one with the second most votes nationwide, so he's the obvious choice as the alternative to Trump. He'll point at all those votes he got, and all the votes for him will look the same — the people who love him and want him to be President and the people who voted for him only to stop Trump and never liked Cruz at all.
You can't mark your vote to register in a particular way. You can't make it say: Cruz, but only as a device to stop Trump; don't take this to mean I support Cruz. And what if you could? What if Cruz were asked to fight on, undercutting Trump, because he's the only tool available for the job, but he needed to understand that he will not get the nomination. He's not wanted. He's really fighting for someone else — Paul Ryan or Mitt Romney or (I've even heard it said) Scott Walker. If Cruz knew that's what he's really doing, could he accomplish the designated task and take down Trump? Shhh. Don't tell him. He's just our tool.
But many of us voters would like to be able to mark our ballot: Cruz, but only as a means to the end of defeating Trump. Don't take this as any kind of an indication of support for Cruz.
And I realize that this is an iteration of what is always true: The vote is inarticulate. You're only able to vote for someone, not qualify it in any special way. You can't make it say: I don't like half the things you're proposing, but you seem like an honest man. Or: I don't believe a thing you've been saying, but I think you'll be a savvy pragmatist once you're in office doing the job. Or: These candidates are all disgusting, but you're the least disgusting.