I’d look at it like this, in a system like Australia’s with instant run off preference voting, you rank your preferred candidates from 1 to x. If no candidate wins an outright majority in first preference votes, the candidate with the least is removed. Then the remaining candidates are tallied again, this time with all their first preference votes PLUS any second preference votes they were given. If no candidate attains a majority, you again remove the lowest voted for candidate and in the next round, the remaining candidates get all their first and second preference votes, plus third preference votes. The process repeating until a candidate has a majority of votes cast in a given round of instant runoff voting.
So, let’s say that we hypothetically had this system in America. If your “first preference” vote is Jill Stein, and your “second preference” vote is Hillary Clinton, and your “third preference” vote is Gary Johnson, and your fourth preference vote is Donald Trump [Australian IRV you must vote preference for every candidate listed, so you cannot churlishly say, “I NEVER prefer Trump, and will not number him!” if you leave the box next to his name blank, your entire ballot is thrown out.] Now, since we don’t actually have this system, you have to choose one candidate on election day. Let’s say you choose Jill Stein. In essence, you’re “working against your interests.” Because while Stein was your first preference vote, literally anyone with a working brain knows she can never win. So in effect, your “second” and “third” preference candidates, Hillary and Johnson, are actually being “denied a vote” that could’ve gone to them, and instead went to a candidate who can never win (Johnson also can’t win obv.) So in this effect, since Trump is your least preferred candidate, and you’ve voted for a candidate who cannot win, instead of a higher-than-Trump preferred candidate who can win, you’re essentially denying said candidate a vote.
That isn’t quite the same thing as voting for Trump, but it’s similar to a Hillary supporter “spoiling their ballot” by filling it out incorrectly, which makes it easier for Trump to win in a FPTP system (there is one less vote out against him, so the # he has to hit to “be first past the post” is lower.)
This is also true if you flip it, if Hillary is your least preferred candidate and say, Ron Johnson is your most preferred, a vote for him is still hurting a guy (Trump) that you prefer more than your least preferred candidate.
I like instant runoff voting, and I also wish more Americans could “think this way” even though our system doesn’t work this way. Instant run off voting forces people to think about their preference instead of just making a “singular choice.”