He can't feel anything, oddly enough, and for a second, nothing happens.

And then, slowly, he can feel that arm begin to dissolve. It's an oddly painless sensation? If he's watching, he can see that it's mostly like his fingers are fading into glittering particles, blowing away in some unseen wind. There's no blood, no exposed muscle or bone... despite the effect, it's more like someone has just taken a giant eraser to him and is slowly scrubbing the existence of his hand away.