The Windows Insiders app give Windows Phone users the ability to try pre-release operating system updates before they are pushed out to the general public. Recently, Microsoft pushed an update for the Windows Insiders app, but there's no change log available at the moment.
The app description says,
The Windows Insider application provides registered Insiders the ability to receive pre-release OS updates on their phone, directly from Microsoft. For more information about registering and becoming a Windows Insider visit http://insider.windows.com.
It's been a few days since Microsoft released the technical preview of Windows 10 for phones, but the list of supported devices isn't too big. There's a workaround to get it unofficially on your device, but we wouldn't suggest doing it as there's a risk of bricking your phone. Reports suggest that Microsoft has patched the workaround, restricting users to install the preview on supported smartphones.
The high-end Windows Phone devices are not yet supported, but lets hope Microsoft will add support for them in the next build.