Earlier this week, Microsoft released Windows 10 for PCs build 17686 to the Fast ring. The build comes with a small surprise though, because there's a new option in Settings to 'Switch to S mode' (via Thurrott). If you search for it, it's there, although it doesn't do anything at the moment. It just takes you to the Windows Activation page.
If it's a mistake, this wouldn't be the first time that something like that happened. We first heard about S mode for various versions of Windows 10 from a Bug Bash quest. The quest was later removed, before Microsoft formally announced the feature.
But when it was officially announced, the company only said that you'd be able to purchase a new PC in S mode, and that you'll be able to upgrade to a full version of Windows 10 for free. In fact, the word has been that there wouldn't be a way to switch back.
It's also unclear how switching back to S mode would work. After all, S mode doesn't run untrusted Win32 apps like Chrome and Photoshop, so those apps would either have to be uninstalled upon switching to S mode, there would need to be a factory reset, or they'd just have to be blocked from running.