Interesting, and very well done. She wouldn't think much of my left hand position, which is admittedly strange due to limitations imposed by a table saw accident many years ago and the approach taken by the surgeon to the repair.
What she has to say about the left hand A key usage is also valid for usage of the right hand upper joint B-C trill key. One of the side effects of my range of motion hand position issues is that I offset the holes slightly so that the left hand pinky keys (on the lower joint) rest slightly higher than their typical position. A side benefit is that it also places the end of the B-C triller right next to my index finger knuckle if I rest the finger along the body of the clarinet
, with the fingertip still near its hole, instead of lifting it to reach the key, so crossing the break at B or C is just as easy as crossing it at A. Essentially the same technique as what she's describing in the video, but with the other hand. Should work almost as well if you align your joints in the standard way.
Yes, I use that key a lot, which is why it occasionally occurs to me to shift the break location. On my Alberts, the key plays in tune, unlike on Boehm's I've played. And for it to work easily requires the 3 trill key setup. The fourth key, if you have it, gets in the way.