WinBeta found a new screenshot of an internal Windows 10 build. They originated from the same group, Wzor, that leaked almost every other image from unreleased Windows 10 builds. The only real feature that is shown is a translucent start menu. To make the transition a little less jarring, you are able to partially see the content behind it.
This feature should be especially useful for the full-screen start menu, so that it looks like an overlay, rather than: “Your computer is doing something totally different now!” You can still see, if only a little bit, what you were doing. It should feel a lot more like the Steam Overlay rather than a full context switch.
The build is also not labeled Microsoft Confidential, so it might be on the branch that is designed for public release. We are due for a new build, so it should only be a matter of days before consumer previewers, and apparently enterprise ones too, get pushed forward… … after about five-or-so reboots.