In Contrast to the Normal Testing

Caves & Cliffs: Part II is in development at Mojang as the next major Minecraft update and you can test the new world generation is a special build.

It's a significant change in the way Minecraft is constructed, which is why Mojang chose to test it in a different way. This Snapshot is not available in the launcher, unlike regular testing. Instead, if you head to their explainer blog post it gives you some instructions and a download link.
Another Blog

It's possible for things to change but here's what's the difference:

 New world heights and depths New mountain biomes (lofty peak, snowcapped peaks , meadows, groves, and snowy slopes). - New cave formation (cheese and spaghetti caves) and local water levels. Cave biomes are created naturally underground and within mountains (lush caves or dripstone caves). - Natural variation in the shape of terrain and elevation, independent from biomes. For example forests and deserts could grow up on hills without the need for a specific biome.