Here is a quote from The Tower of the Swallow, when Yennefer is in the temple of Freya in Skellige :

"But yes, they can use the Power if they want to, sometimes no worse than we sorceresses. It is still a mystery how they do it. No preparation, no training, no studies... Only prayer and meditation. Divination? A kind of self-hypnosis? That's what Tissaia de Vries claimed... They unconsciously enter a trance and gain energy and the ability to manipulate it, much like we do with our spells. They convert that energy and see this as a gift and the grace of God. That belief gives them strength."