Though screencasting or mirroring has been an integral part of Nexus 5 ever since Chromecast was introduced by Google, the new AllCast app simply extends this functionality to other Android devices by bypassing the need for WebRTC/VP8 encoder requirements.
Screencasting or mirroring basically allows users to cast over an Android display to a computer running the Chromecast Receiver extension.
Now this functionality has been modified by rewriting the Mirror to Chrome feature to use h264 decoder software (instead of WebRTC/VP8) via Broadway.js.
What This Means to Android Users
Basically, any handset that can run the AllCast app should be able to support screen mirroring. However, the handling and user experience might differ from one device to the other based on its hardware configuration and Android version installed.
Interested users may take a look at the complete source code as well as download the new AllCast app for free here: https://github.com/mbebenita/Broadway
Check out Koush's interesting demo video on Android to Chrome mirroring with h264 decoder (below):