Outcome manipulation is Siobhan's subtle but powerful ability to change an event or course of events by visualizing it with hard concentration. It is recognized as one of the most powerful abilities in existence.
Though Siobhan did not believe she is particularly gifted, Carlisle Cullen believes she shows this ability and can manipulate the outcome of situations, by wanting them to go a certain way and concentrating hard enough on this desire. Although Siobhan herself does not believe she possesses the gift, Stephenie Meyer marked her name with an asterisk to show she has a "quantifiable supernatural talent" proving she does have the gift.
This gift shows great potential. As in time, and with practice, Siobhan could possibly learn to manipulate events with little or no effort.
For this power to work, it requires her full concentration.
During Breaking Dawn, Siobhan wishes for the confrontation with the Volturi to not turn violent. And throughout the confrontation, the situation remains peaceful as she has visualized, with only Irina as the casualty for providing false report to the Volturi. By the end of the saga, she finally comes to believe in her power.