This dōjutsu is more of an involuntary ability. It allows Shion to predict when someone is going to die. While the predicting is in process, her eyes take on a sacred geometric pattern, shifting somewhat like a kaleidoscope.
The actual effect is allowing her soul to return to the past when she dies. When she does this, she informs her past self of the event and the people around her when this happens. She then tells one of these accompanying people that they will die. This prompts them to give their lives to protect her, effectively changing the future by trading their lives for hers. This therefore allows her to "predict the future" by partially controlling its outcome. She has been wrong only once, when predicting Naruto's death, where she protected him instead.
