So here are the time stamps you are talking about:
1.47

2.34

So the idea with 1.47 is to really overemphasise the stretch of the connective tissue (fascia if you like!) across the yin channels. This is achieved by moving the body beyond the range that would be considered optimal (say for a martial function). So I'm leaning back a bit and losing alignment of the body, just to get that feeling of a stretch.
I just tried it again, and I don't find that this posture has any effect on my abdominal region, or prohibits me from using it for reverse breathing. (In fact, I might even conjecture that because you can't access the lungs in the upper chest as easily (they are being squashed/stretched) it should make it a bit easier to use abdominal breathing?)
Once you get that feeling you find using the reverse breathing has a notable effect on the pull on the yin channels (your arms should move a bit just because of the breathing - which you should be able to see in my video).
There's a separate exercise for the yang channels, then later I say that once you get the hang of this, you can forget that posture and move on to doing it in the standard hug the tree posture at 2.34.
Again, the extreme posture of 1.47 is just for beginners to get the feeling. It's a progression.
As to whether it's a "trap" for beginners... I guess that would depend on the person? Hard to generalise about everybody like that. I find some people just get everything right straight away, and others tend to somehow manage to find a way to do everything wrong immediately! The only way to see if it works for somebody would be for them to provide some feedback either in person, or via a video.
---
Incidentally, once you get the idea of breathing affecting movement that I'm outlining in this video it starts to reveal what exactly should be going on in ba duan jin exercises. But that can be further complicated by other factors, and I don't think there is only 1 way of doing the ba duan jin - for example, you can stretch your arm - hold the stretch, then use breathing to increase the pull down a channel as a qi conditioning exercise, but don't move the arm. That's a different method that I'm showing here of allowing the pull on the channel from reverse breathing to lead the movement. So the situation is complex, and probably only solved by asking your teacher what the exact purpose of each ba duan jin exercise is in their system.
(That's a discussion/method that is beyond the scope of my course).
----
