Describing the right posture for aikido should be doable, shouldn't it? Perhaps attach a picture? When you do these exercises, how do you check your posture?
I do these exercises on my own late at night after putting the kids down to bed. At that moment, it is difficult for me to check myself beyond my bodies internal position feedback. A mirror would help I guess. However, I am doing exercises that are part of my regular aikido training, where I get feedback from my instructors.
As for illustrating posture. My model for bokken, and an important model of mine for posture in general, is Claude Berthiaume Shihan. He can be seen teaching bokken suburi here:
I especially like how straight he holds himself and how grounded he is.
Another model of mine is Harvey Konigsberg Shihan. I consider him a great example of staying relaxed and dropping the shoulders. here is an example:
How do you guarantee/verify that the sensei and you are talking about the same things? If I'm allowed a cynical interpretation: the sensei restricts himself to saying vague stuff, knowing that every student will at one time feel something of which the student will decide that it's what the sensei was trying to explain. And the students that don't experience such moments, will just think they're not good/sensitive/intelligent/etc. enough to get it.
I have had instructors that tended to be on the more nitpicky side of things and that spend a lot of time trying to show and explain details. I never said they restricted themselves to vague explanations. It's just that some things are hard to communicate though words, especially when there isn't a known shared experience. When the istructor telling you to relax is the one that showed you the exercises you use to develop this, when you've been both nage and uke with the instructor for the paired exercises, it is at least possible to hope you are talking about the same thing.
Also, the sensei should be able to see/feel if you're doing it right, so that when you think you feel something, it can be paired with the sensei going "yeah, that's better". I've had my instructor demonstrate a technique while exagerating my mistakes (often posture related) to show me what I was doing wrong. Then there is the pressure test. Often, the biggest, strongest guys have the most trouble with kokyu-ho because they can muscle past most people rather than using the more relaxed integrated power that I feel the exercise aims to develop.