Insiders will notice a new option in the Insider Program area in the Settings app that provides a drop-down menu with two options. By default, the option is set to "Active development of Windows," which tells the Insider Program that you wish to continue receiving preview builds from the Fall Creators Update development cycle. This option will only be present until the development of the Fall Creators Update is complete, which should happen around mid-September. Once development on the Fall Creators Update is done, Insiders in the Fast ring will then be automatically switched to the Redstone 4 development branch.
The other option, which Insiders will have to manually switch to is "Skip ahead to the next Windows release," which does exactly what you might expect. This option will put you on the Redstone 4 development branch now, ahead of the Fall Creators Update being finalized. This means that you will skip ahead to Redstone 4 development, and not be able to test final Fall Creators Update builds as they roll out to Insiders for testing.
At first, Insiders who choose to skip ahead will receive builds that are practically identical to the builds that roll out to Insiders who are continuing to test the Fall Creators Update. That won't be the case forever, and when Redstone 4 development ramps up, Insiders who skip ahead will start receiving builds from a much higher range of numbers, which will include more bugs and eventually new features.
On the flip side, Insiders who remain testing the Fall Creators Update will see fewer big features and more bug fixes and stability updates as the preview program progresses, as we're at the tail-end of development of the Fall Creators Update, which means Microsoft is now focused on making sure the Fall Creators Update is as stable as possible for public rollout.