Left 4 Dead 2 is a mod for the game that allows you to play with interactive features from your viewers. For example, they can spawn zombies for you or help you with weapons. You can find all available commands in the list below.
How to install it?
Launch the program and go to the Presets section.
On the right side, you will see the preset menu. Find the game there. Alternatively, click “All Game Mods” and locate the game in the list.
Click on it and scroll down a little. Press the “Copy this preset” button.
You can customize the preset to your liking, then click “Save Preset.”
Now open Steam:
Go to Properties and enter -insecure in the Launch Options.
Start the game, select “Campaign” → “Single Player” → “Start.”
Launch the game and enjoy!