A button you press in Twitch Dashboard when you are offline that makes it so that you enter the testing mode. In testing mode, you can see your stream as if someone else was but without actually going live to the public. To show that you are in testing mode all the Twitch stats Will change to say "Testing mode". You will be able to see bitrate stats and change how your channel looks. It would also be cool if you could get a shareable link that you could give to your friend so that he can see what it looks like, you could give him access to edit stuff or just to watch. With this, you could see how your stream quality is and how all your panels and stuff looks on for example mobile or TV.