"During the ritual." Istredd assumes that's what he was talking about, that is what happened to people who were taken. Sometimes one or two at a time, like him and Claude. He knows how much the ritual damaged people, but he also knows that it could set off abilities. Ciri's scream is proof enough. It could have been her fear too, or it could be whatever happened to them.

"It isn't unusual for people to be able to tap into their abilities in an emergency with a refined skill they're not used to. It's part of the fight or flight reflex, your body and subconscious can react faster than your mind." And in this case, his mind wasn't working anyway. So his instincts were driving him, and that made it easier to tap into something he wouldn't ordinarily. Or at least that's what it sounds like.

"And you being able to maintain a flame continuously while on the mountain is very impressive." They already knew that he had a more comfortable time using fire that was already created. It's being the source of it that he struggles with.