Outcome manipulation is the theorized ability of Siobhan's to change an event or course of events by visualizing the outcome with hard concentration. Though she doesn't believe it herself, her friend Carlisle Cullen does.
It is recognized as very subtle but also 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.
An example of Siobhan's power is her determination to end her creator, Sancar, before the end of her newborn year and which came true. When Maggie first joined her coven, Liam (Siobhan's mate) was very territorial, but Siobhan willed it to work out and it did in the end. Carlisle Cullen theorized she had the gift when they became acquainted with each other.
It is also theorized that Siobhan had this power as a human, which was what allowed her to do what she pleased; driving away the men whom she thought she did not need, and managed to build a life for herself and her mother.
- "... She has a way of deciding her goals and then almost... willing them into reality. She considers it good planning, but I've always wondered if it was something more. When she included Maggie, for instance. Liam was very territorial, but Siobhan wanted it to work out, and so it did."
- ―Carlisle's description of Siobhan's potential power.[src]
During Breaking Dawn, Carlisle persuades Siobhan to try and visualize a peaceful confrontation with the Volturi. Minutes before the confrontation began, Siobhan does so, most likely wanting to try anything possible to resolve the situation peacefully: 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 comes to believe in her power, naming it the power of "wishful thinking". When Carlisle and Edward theorized that the Volturi will return for revenge someday, Siobhan jokes that they cannot fail to stand against them "as long as she wishes otherwise".