To use this feature, you have to tap on video icon which will be available at the top-right of the messenger conversation. This will only work if both the users are in the conversation at the same time otherwise it will not work. Once the icon is pressed, the app will start to show your video to the other user. Sound is off by default but you can turn it on if you like. The live video will be shown to other user and he/she can watch you or they can also add the live video of their own.
The videos of the both the users will continue to stream and the chat box will be still available so you can also easily chat with the person on the other end.
What are your thoughts about this new feature from Facebook?