What exactly does the "Cache to disk" button do?

[pixelminer]

It says "Enable caching KD-trees to disk" but I have no idea what that means. Does it make things render faster some how?

phong

[dougal2]

If you launch the scene to render for a second time (for example, to resume a render, or of you changed a few small details in the scene), indigo will start up a little quicker if you enabled the cache the first time you rendered the scene.

(phew, long sentence)

basically, it'll speed up indigo's startup in some cases, but not the actual rendering speed.