A voter votes X equal to Y if the voter doesn't vote X over Y, and doesn't vote Y over X, but votes X over someone, and votes Y over someone.
A sincere vote is one with no falsified preferences or preferences left unspecified when the election method allows them to be specified (in addition to the preferences already specified).
One candidate is preferred over another candidate if, in a one-on-one competition, more voters prefer the first candidate than prefer the other candidate.
Statement of Criterion
If a majority prefers one particular candidate to another, then they should have a way of voting that will ensure that the other cannot win, without any member of that majority reversing a preference for one candidate over another or falsely voting two candidates equal.
Schulze (with winning votes as the measure of defeat strength), Maximize Affirmed Majorities, Bucklin voting, CDTT methods, MDDA, and Condorcet//Approval comply with the Strong Defensive Strategy Criterion, while Approval voting, Cardinal Ratings, Borda count, Plurality voting, Instant-Runoff Voting, Raynaud, and Descending Solid Coalitions do not comply.
Compliance with SDSC means that a majority never needs any more than truncation strategy to defeat a particular candidate, even when countering offensive order reversal by that candidate's voters. Offensive order reversal is the only strategy that can create the need for defensive strategy in Schulze.
A very similar criterion is the Minimal Defense criterion.
Some parts of this article are derived with permission from text at http://electionmethods.org