I think I was concentrating too much on being able to go low
You know this is a very important point. This thing about going low is like a plague. I just mentioned this before to somebody..I think it might have been Marv.
You should never be working at the end-most of your range of motion. That is the point at which your are weakest and have the least control. The idea that you must go as low as possible in everything you do or display the most ROM possilbe leads to the kind of thing in the vid. You only need to go to the point where you can display perfect form and where you have full strength. Once you go past that you are not only at a disadvantage but your are basically no longer exercising but stretching under load (or at least trying to resist as stretch under load).
I think maybe a lot of people don't recognize that sometimes thing feel so heavy because they have exceeded their effective ROM. For instance your do static split squats and you try to go as low as possible with a long stride length so that the hip flexor is stretching bigtime and you get a ham stetch and what is happening is that you are no longer doing a split squat you are doing a loaded split!
As your ROM in terms of flexibility and mobility gets better the trick is to be able to increase your EFFECTIVE range of motion. Ever heard that term? Effective ROM? That is what it means. If you do a full static stretch then you are not at an effective range of motion.
Another great example of this is when people do deadlfit variations like RDL's. They like to "get a good stretch" in the hammies becasue they have been led to believe that this will make them grow. The way to do an RDL is you go until you feel it in your hammies. You feel a slight tug. People have taken that and turned it into "stretch the hams". Well, if you take that too far you are comprimising strength and risking injury. If you can increase your ROM with control that is different...but not always necessary.
So..the moral is only go as low as you are effective. In this case it means only as low as your back remains set.