Note that this version of the tool isn't finished so many models in model viewer are displayed without textures, if you need to preview models you're better off with previous versions of the tool.
For how to use the tool, see this post and also this post
Regarding the new features, its mainly the ability to, add/remove copy/paste, insert, delete blocks of data within the JSRF file container structure.
For example you can select a model or texture from and click "copy", open a different file, select a texture and click "paste" to overwrite by whatever you copied earlier.
Also, here's some mods:
That's all for now ;)-neodos