Truncation is when a voter votes honestly, but doesn't show some of their lower preferences. In the extreme case, it becomes bullet voting, where the voter only indicates their first preference. It is often done as part of a voting strategy, although it can also be done to express indifference.
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A variant of the Participation criterion or Independence of Irrelevant Ballots offers one way of justifying Range voting being no worse than FPTP: voters can never be hurt by casting an FPTP-style vote (bullet voting) relative to not voting. Many other voting methods, such as IRV and STAR (and possibly Condorcet fail even this weaker version of Participation. Example for IRV:
30+2: A>B 31: B 49: C
If 2 of the 32: A>B voters show up and bullet vote A, then B is eliminated and then C wins. But if they don't vote, then A is eliminated and then B wins.