If you think of it in terms of all of your mind and bodily processes as signals, some signals beget others. By harnessing the breath and "putting your awareness on the physical mechanisms of the breath" you are taking through-stream-energy and focusing it on a physical process. By harmonizing the breath and focusing the awareness properly, it makes the process create more energy than it consumes. By focusing and harnessing that thought-stream-energy, you are "training a path of least resistance that your core energy overflows into" when the body isnt using it for anything else. By attenuating some signals (e.g. breath) others become more apparent (e.g. heartbeat) and you have thus lowered the "noise floor" of all the body-mind processes. Dr Yang and other have written about the high qi-oxygen utilization of the brain; so the idea is to create a path of least resistance for all of these additionally available energy potentials the body isnt using - and since it is also focused awareness, not just a low consumption state of oblivion, you are directing where some measure of that energy goes - how efficient that process takes place is a function of the level of stillness you are able to obtain. This stillness is spontaneous and effortless, and doing the breath focus is a sure fire way there if you cultivate the technique.
I was reading cleary's golden flower translation and a line read something like, "if you suppose that you lose energy through expenditure, it is only because you gain nothing in stillness."
Dont worry about your overall level of progress, just focus on one or a couple simple things at a time and you make progress faster, the whole get out of your own way thing, you cant think your way there. Learning to redirect your thoughtstreamenergy to a certain extent cures you of things like boredom - how can one be bored when its just a moment?
ZZ is awesome, I know a few people who have made more solid progress only doing that. But when you add in meditation and attain good stillness, you basically "soak up more" of that which you cultivated earlier. I think I have written about some of the physical particulars in some of these adjacent threads, but if there is anything in particular let me know.
Dont be afraid to make progress! That's why safeguards like a well established path of least resistance is indispensable, you revert to training when you've had enough.