It just doesn't get hot. If I remember correctly, there's a massive heatsink there that distributes it all, and having an aluminum body probably helps as well. Either way it's the most comfortable laptop I've ever used for this exact reason (well, and the humongous haptic trackpad).
Yet it's still fast enough to do development on it, never mind more mundane stuff like web browsing or watching movies. I'm not at all an Apple fan, but when people say that M* silicon is almost magic, they're not wrong.
It throttles itself. You can install a thermal conductor on the package which makes the whole chassis into a radiator and the thing gets too hot to touch under load, but you get more performance.
