This is to continue on my previous thread of learning to connect the body with ground and gravity strengths (Jin in Chinese, often bundled into the ki kokyu skills in Japanese, though I've heard the term Kei used as well). There are three more topic areas I eventually want to cover - opening and closing the body as a fundamental method of movement, connecting the body via intent-posture-breath training, and developing the middle as the governor of connected intent-driven whole-body movement. This post will focus on the open/close part of the body movement as a function of internal training.
My opinion, but I'd argue that initially training the open/close happens somewhat organically in a movement such as tankan/tai no henko - in that there's an irimi entry which can be a big opening of the body, a closing of the body as the irimi moves into tankan, then another opening of the body in the turn and redirection. Isolating that open/close whole body movement, you can begin to train a coordinated whole body method of moving everything together at the same time.
This whole-body coordination should be something that people are already practicing in whatever martial art they're doing - even if they aren't on a path to be able to build intent-management of ground/gravity forces, or connecting the bone-muscle-tendon-tissue elasticity that transmits and enhances the other powers. So within this coordination exercise, it's worthwhile to visit the open/close frame of movement as the building blocks of every single other movement - there's reasons for this for developing internal strength which I'll touch on.
As you're developing the coordination of the body's connection to the ground/gravity for sourcing your power, you can see aspects of opening and closing movements that more naturally will extend the sourcing of the ground and/or gravity as the primary strengths when effectively transmitted with the whole body. For instance, in an opening movement of the body, the ground will be the primary up to a tipping point, at which time you'll have reached the logical extension of the "open" and it will be time for the body to "close" into the next movement, at which time the downward gravity power will take precedence. Virtually every technique can hang on these assumptions (building on the vector movement bucketing of Taikyoku Aikido as developed by Ellis Amdur and taught at a number of mainstream and independent aikido facilities), so that as you're rewiring your body to more effectively move with IS, these tools can inform where you're moving "as always" vs. "increasingly more correct via intention and connection".
To tie in the other pieces that will be future threads, another reason for the somewhat dogmatic approach to this whole-body open/close, is that in order to correctly combine the intent-forces with the increasingly connected body - the reverse breathing on the closing movement will create a mechanism to load the tissues and stretch the elastic connections, thereby storing tremendous potential power that doesn't look like much from the outside. There's other mechanisms, of course, but this is an important gateway milestone in development that it acts as an important checkpoint to begin to bridge the intent-force with the body connections (the qi/ki and jin/kei so to speak, via kokyu). The additional steps of the middle as governor gets into ongoing development and making sure that the middle can move freely to stretch and direct the intent-force management through the tissues in a way that moves all of the body parts together as a single unit - most efficiently again by opening and closing the body.