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.
Thsi gift shows great potential. As in time, and with practice, Siobhan could possibly learn to manipulate events with little or no effort.
This power requires Siobhan's full concentration in order to work. Although the situation may work out according to her intentions, unexpected incidents may still influence the outcome, such as Irina's punishment.
After the passing of her father, Siobhan learned to become self-dependent and profited for the family, which only consisted of her and her mother. Every man who ever pursued her was turned away. Supposedly this was a result of her power when she decided to live without a man in her life.
An example of Siobhan's advanced 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 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.
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. Thoughout the confrontation, the situation remains peacefull as she has visualied, 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".