When i do practice, I tend to think in general terms. Don't use the shoulders to raise the arms is the main thing for me. There are a few other things I play around with though, and I'll try to organize them here.
I should begin by saying that I haven't been a good student for quite some time and that I have only the most superficial understanding of anything (whatever I might be said to actually have an understanding of) to begin with.
On the whole, my focus is on balancing tension where ever I notice it, and trying to have a full, relaxed feeling.
Right now I begin by trying to have a strong extention along the vertical axis (spinal alignment); a feeling of "good" posture, reaching up through my crown and down through my perinium. So I start with an attempt at having a full sense of vertical expansion ("expanding my central column"). For this I try to reach up with my crown then "dangle" my body from it, then I bounce back and forth between this dangling feeling and the pushing up from the hips/legs feeling until I can have a sense of both at the same time, some times better than others. I also try to have a feeling of "sitting back within myself." Once I'm satisfied with the relaxed and expanded feeling I'll start playing with powering my arm movements by it, using my legs to lift and drop the column for cutting and raising movements, trying to use momentum more than muscle. I do a kind of "reverse furitama" where I reach through my arms and fingers and try to raise them with that up and down movement of my central column, usually bouncing my arms off my body (tricepts bouncing off chest area). For this I try to feel like my hara leads the movement: hara up pushes arms up, hara down pulls arms down. I try to time it right so I'm pulling down before gravity does.
For my upper body more specifically, I constantly try to feel my shoulder girdle pressing down until I get a sense of it resting on my spine/central column (and keeping it firmly resting there as much as I can, since once I start moving it tends to not rest "down" quite so well). I try to have my shoulders balanced with respect to the spine (e.g. my left shoulder and chest area droops and is somewhat concave compared to my right side). I try to keep my humorus drawn into the shoulder socket and pay close attention (as close as I am able) to whether or not it's being seperated. I also try to feel all the way around it at the same time. In fact this is the basis for my meager practice: sequential feeling of contiguous lines and areas. If I feel for my shoulders as I practice cuts, for example, I notice I'm usually just feeling portions of them; some more than others, and some not at all. My efforts are centered around feeling the whole thing at the same time (one thing moves everything moves?). Sometimes I feel something like a band running around the shoulder joint, more or less vertically, but I'm not sure if that's because I'm using too much latisimus dorsi or trapezius or whatever it is I'm using.
As usual, the more I write the less confident I feel. Like I said, i don't practice enough...and I probably shouldn't try to chime in because of it, but FWIW, there's a start. I might get ballsy and shoot a video of what I look like. I'll try to think more about it and see if I can't find a better description...might help to practice more too...have I mentioned I don't practice enough?
What do you tend to focus on?
p.s. in my defense I'm juggling my 2-year old, my 5-day old and this post, so I apologize for any haphazard descriptions.
Well, it's been a fun year. I have spent much time on trying to decipher "drive your spine into your arms" and "drive your spine into your legs" to be balanced with the koans "no sharp angles" and "you don't see naked skeletons walking around", and I didn't directly hear this one, but I think it was implied at times just by how spectacularly I managed to screw up the first two, "let's step back a moment, first try to drive your spine into your spine", actually phrased more like "no no, you're using your spine like a giant stick" , until it eventually started to sink in that it is all one drive and that it went, well, through things. That was a lot of quality time pushing walls in all number of variations - sitting on butt, on knees, facing wall, facing away from wall, shoulder against wall, arms against wall, head against wall, butt against wall, back against wall, with arms collapsing, with legs collapsing, with neck collapsing. But within each of those it was working on looking for and diagnosing collapses at specific points in the body - like pushing the head against the wall while sitting on butt for trying to monitor collapse of the cervical spine. Lots of pushing people while they are pushing on me just the same - I guess you could say sumo style - to test all that against real resistance - on the ground, standing up, on the knees, sitting down, gripping the arms, gripping the neck, gripping the legs. Then lots of practicing that in a wild variety of movements trying to make sure nothing got lost in translation, and still a lot of doing that, but for almost different reasons now.
Somewhere along the line came the realization of Mr. Armsly, that he is really Mr. Legsly, and that Mr. Legsly is really also Mr. Armsly - there is really quite a striking and beautiful symmetry to the pelvis and the ribcage, and it really still sort of amazes me. But going back a few years ago, I don't think I realized I could have and should have worked on all of that, and maybe all I would have worked on was zazen, funekogi undo, irimi-tenkan, suburi, stuff like that, and looking back on the decade of trying that, it feels like I got more out of an hour sitting on the floor pushing the top of my head into a wall than all of it, so I am starting to think it is less a matter of simply time-in than a matter of time well spent.