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.