So he used what little he did have to push his very magic to the surface of his skin, hoping the heat would melt the ice. It did, but whatever this draining spell was that Dumbledore was using, it now no longer had the ice barrier to Harry's actual body. It hit him, hard. Squarely in the chest, smashing his ribs to pieces. He shrieked in mind-breaking agony. The spell was sucking the last breath from him, and some of his life, too.