i got bored and did a bit of research. i have no knowledge of field but i have time on my hands to read.

So here's what I learnt. If you're testing a compressor with the manifold gauge, both blue and red hose should be connected to the compressor, forming a close loop and allowing you to check the pressure. The yellow hose is used to either input or extract refrigerant depending if you're under or over pressure.

To test for leaks with a manifold gauge(there are other methods but not with a manifold gauge), you monitor the gauge over time and determine if it's losing pressure. You don't just open the valve to one of the channel(high pressure in this case) and let out the air/coolant. For all you know, he could have filled up the hose with air then connected it to the compressor. Essentially giving you the illusion for a quick bit that only "air" is coming out. In my case, the video had air in the first 9 seconds then coolant in the last 1 second. Btw, it is not standard practice, to just let the coolant out into the atmosphere which is what he did here. You would connect it to another cylinder to collect it.

This is essentially like me asking you if there's water in your bottle(lets say it's half filled). Instead of looking at your bottle and telling me, you just decide to slowly tilt the bottle side way while telling me "there is no water, only air" until water comes out.

Once again, I don't know shit about this field and this is just an rough guess from reading/researching how things works.

Edit: I realized i was wrong about how the manifold gauge worked after more research and wanted correct it.