Have been getting instruction to 'bend my elbows' , 'loosen my knees', 'bring my bottom more under me' and\or bring my shoulders back.....
From the video (and my own feeling of what my body was or wasn't doing) I could see that actually all of these instructions actually meant that by lower torso wasn't 'pushed' forward and down.
My hips, or more particularly my hip sockets for my thigh bones are much wider than they had been and these in turn push my thigh bone down (the visible change to my instructor is that my knee is loose, but this isn't what my feeling is).
At the same time my core torso muscles are working a lot harder - they are pushing (and I mean pushing) my torso from hip to thorax forward and down.
The combination of movements also involve my glutes (butt muscles) to ensure that my pelvis doesn't over-rotate they must hold some tension in some quite deep muscles over the outside of the hip socket area to secure the seat bones on the saddle.
I believe that this is what Mary Wanless refers to as the pinch - but I have always disliked that term and even now I can't deal with it. (Certainly the muscles pinch ouch! if you go too hard the first time)
My best attempt for an analogy is the limbo - you know when you limbo you stand wide with your feet and hips and push your stomach\torso under the bar.
Once I have used this torso push\hip stretch my shoulder\hip\heel is always correct (without me attempting to throw my shoulders back or other disruptive behaviours) and my back remains supple because it is my abdominal muscles doing a lot of the work rather than me fussing about trying to force my spine to take this angle or that...
Now in writing this I find myself in the same dilemma as an instructor trying to explain what I have physically done