When a ray hits a brick that has not yet been loaded on the GPU it will add the request to a request buffer. The CPU will then upload the brick to the GPU. This way only bricks that lay on the surface of a superchunk will be loaded since rays won't penetrate into the inside of a superchunk.
Brick streaming is described in this repo:
When a ray hits a brick that has not yet been loaded on the GPU it will add the request to a request buffer. The CPU will then upload the brick to the GPU. This way only bricks that lay on the surface of a superchunk will be loaded since rays won't penetrate into the inside of a superchunk.