Now you need to adjust the airflow.
Notice if you cut the airflow almost completely off, the skimmer body will only have a few bubbles inside. Go ahead and slowly cut down the air flow until the bubbles have stopped.
Now notice that you can open the air intake line slowly and see the bubbles. Take note that as you open the air intake line - more and more bubbles will be produced and the skimmer body will look like white foam inside.
Keep going with the air intake. Open it up even further and further and further. There will come a point where the line is open TOO FAR and it will actually FOUL the impeller blades. You can see the bubbles reduced to lower amounts in the skimmer body. At this point, the impeller blades are barely pumping any water at all. They are too fouled with air to do much actual water movement. The other issue is the skimmer body is usually NOT a white haze of foam at this point. Most of the time there is only a thin column of bubbles coming out of the pump tube and into the skimmer body.
So take care and OPTIMIZE the air intake line. Spend the time to properly adjust it for maximum bubbles WITHOUT fouling the impeller. I always liked to spend the time and find that sweet spot where it is producing so many bubbles that the entire skimmer body looks like it's filled with foamed milk.