The second Insider release on the “Redstone” branch has been pushed to Fast ring users. Once again, this has basically no release notes because a lot of features are “under the hood.” The push with Windows 10 since just before the holidays is to create a sensible structure for various teams to target with their changes. You could imagine how difficult this gets when you're dealing with phones, IoT, tablets and convertibles, HoloLens, and high-performance workstations, across a few different architectures.
Insiders who are interested in UX updates and other features will probably be best to switch to “Slow” for a handful of builds once they find one that's stable for them. I can't really see this being useful for most Insiders, because unlike open-source previews where you can contribute to (or develop software alongside of) the internal tweaks, all you really can do is report when something is broken or acting funny. If that's what you want, then it's great that Microsoft is providing these previews.