The study followed 117 teams, each composed of five students working together over a semester. Their collaboration culminated in a team project that was used as the indicator of final team performance, which was expected to show variability alongside levels of task conflict measured by a mid-semester survey. What would lead conflict to help rather than hinder? The study hazarded it would be psychological safety: a group-level feature which is present when members perceive low risks and consequences for speaking freely. Bradley's team reckoned that under these conditions task conflict can remain on-task, rather than triggering retribution and spirals of unproductive negative emotion. This allows groups to reap the fruits of task conflict: more diversity of ideas and deeper exploration.
The results of the study suggest that this account is part, but not all, of the puzzle. After controlling for subject matter knowledge using scores on an exam taken earlier in the semester, the research team investigated the conflict-safety-performance relationship. As predicted, teams that scored highly on the psychological safety measure taken mid-semester showed a relationship between more task conflict and better performance on the final project. But the researchers didn't find the expected drop in performance when teams that were psychologically unsafe conflicted; at least, the decrease didn't prove statistically significant. So in this study psychological safety was shown to have benefits, but not to decisively shift conflict from burden to benefit.
More research is needed to understand harmful task conflict and what influences it. Given the benefits of psychological safety, organisations may want to make efforts to facilitate it, by giving permission to speak out; leaders can role model this, even showing they are prepared to be fallible in public. It's noteworthy that a team may work well and be cohesive without necessarily feeling psychologically safe, so it can be worth evaluating exactly what the conditions are within a group, particularly if groupthink and unexamined ideas would pose highly negative consequences.
See also De Dreu, C. K. W., & Weingart, L. R. (2003). Task versus relationship conflict, team performance, and team member satisfaction: A meta-analysis. Journal of Applied Psychology, 88, 741–749. DOI:10.1037/0021-9010.88.4.741