Re: kokyuho and the reverse breath.
Spine is just a pile of plates with jello in between. From the top of the pelvis to the diaphragm we're all spine plus a soccer ball full of goo. Lateral pressures in particular meet no resistance and back injuries with pushing, etc. Feet connect to the ground, arms to the ribs, and big squishy joins the two.
Breathe in pulls the diaphragm down. Drawing in pulls the perineum up and the abdominal muscles pressurize the whole belly making it much more solid. This protects the spine and is an important part to whole body power. It drives the blood pressure up if practiced too often I am told, and it shouldn't always be needed.
I don't equate abdominal pressurizing with kokyu, it is a piece of it.