What technique does the author use to tell us about Sergei's wishes? What effect does this have?

That first wish, Sergei used up when they discovered a cancer in his sister. A lung cancer, the kind you don’t get better from. The fish undid it in an instant—the words barely out of Sergei’s mouth. The second wish Sergei used up five years ago, on Sveta’s boy. The kid was still small then, barely three, but the doctors already knew something in her son’s head wasn’t right. He was going to grow big but not in the brain. Three was about as smart as he’d get. Sveta cried to Sergei in bed all night. Sergei walked home along the beach when the sun came up, and he called to the fish, asked the goldfish to fix it as soon as he’d crossed through the door. He never told Sveta.