Talk about mysterious, if you actually want to use this particularly on VMWare Fusion, there doesn't seem to be much documentation:
- Google Hangout starts and looks to see what active /dev/video[0-9] are open. That is, if you want this to work, your webcam needs to be active or any synthetic source you create with v4l2loopback needs to be open. This is strangely not in the core user interface, you have to actually be in the hangout itself before you see a settings icon. I haven't yet found the actually underlying settings for Google Hangout but they are there. It is nice that it works with Chromium or Chrome.
- Skype is less opaque as it tells you it is using Pulse audio and that you can actually see the /dev/video0 even if they are not active. So that is less opaque.