From the start the art team on Big Hero 6 wanted to make a really big movie. They envisioned those big sweeping shots of San Fransokyo that made the technical folks shudder. They have a massive rendering farm at their disposal but still probably wouldn't be able to render all the shots in time for the release; for one reason, because the art folks kept making changes, and for another, because they contain so many entities. A handful of folks on the rendering team built a proof-of-concept renderer with support for the global-illumination look they wanted while also being able to handle the massive scale.
As the linked article mentions, the solution was to optimize a massively-parallel algorithm by finding coherent rays that can be efficiently calculated together. It's a batch method: start casting a bunch of rays, identify similar rays and group them, calculate collisions for each group, cast reflected rays, identify similar rays and group them, etc. What I like about it conceptually is that in a way it treats light as a field rather than as individual directed rays.
Their initial tests were rendering an infinite plane of generated buildings. The dystopian metropolis was mesmerizing, and I think Disney would be wise to come up with some dark plot line to set there. Anyway, someone way at the top (the director or art director I think) saw the proof-of-concept renderings and decided that it was the perfect tool to attain their vision of this movie, which, by the way, has a hard release date something like three months away.
So they proceeded to turn the proof-of-concept into a production-grade renderer in time to render all the frames and save the movie. And somehow they pulled it off, they released a really beautiful movie, and they wrote a paper about it, too.