Original sentence: Instant runoff voting fails the Monotonicity criterion, which means that under some circumstances switching a vote away from  a losing candidate to a winning candidate can cause the candidate to lose the election, by altering which candidates make it into the final runoff tally
Edited sentence: Instant runoff voting fails the Monotonicity criterion, which means that under unique circumstances switching a vote away from  a losing candidate to a winning candidate can cause the candidate to lose the election, by altering which candidates make it into the final runoff tally