The respiratory diaphragm is an umbrella shaped muscle situated directly beneath the lungs and heart. It attaches to the lumbar spine and lower ribs and sits directly on top of the liver, stomach, spleen and kidneys.
Respiratory Diaphragm and Transverse Abdominus.
The fibers of the diaphragm attach at the back to either side of the lumbar spine (to the upper two most lumbar vertebrae).
At the sides it attaches to the ends of ribs 12 and 11 and 10 and to the costal cartilage of ribs and to ribs 9, and 8 and 7. (Ribs are numbered from the top downwards. The uppermost pair of ribs are numbered 1, the lowermost pair are numbered 12.) At the front it attaches, via ligament, to the xiphoid process and to the costal cartilage of ribs 6 and 5.
At the back it can be viewed as forming a continuous sheath with the transverse abdominus muscle. It also attaches to the fasia that covers the front of the psoas major muscle and quadratus lumborum muscle.
The Respiratory diaphragm can be viewed as forming a slanting floor for the bottom of the ribcage, turning the ribcage into a bottle like structure that houses the lungs. The diaphragm domes upwards into the volume of the ribcage, much like the bottoms of some types of wine bottles are domed inwards. The line of attachmet of the diaphragm to the ribcage slopes downwards from front to back.
When the respiratory diaphragm contracts, it tends to press downwards, flattening the bottom of the bottle, at the same time increasing its volume. If the ribcage and diaphragm where an actual bottle of wine, the effect would be to increase the amount of wine this bottle could hold. And when the diaphragm relaxes and is pushed back upwards, it reduces this volume, pushing air (or if you are still in the wine bottle analogy, wine) back out.
Because the diaphragm is positioned over the liver, stomach, spleen and kidneys, it actually pushes down on these organs when it contracts downwards. The stomach, liver and small intestines then press forwards on the front of the belly, pushing it forwards when the respiratory diaphragm is contracted downwards.
To reverse this process, the transverse abdominus, the innermost belt like layer of the abdominals can be used to squeeze inwards on these organs forcing them to press upwards against the diaphragm so that the volume of the lungs is reduced. Air can then be forced out of the lungs.
A very basic breathing exercise for learning to feel and control your respiratory diaphragm can be called belly breathing (because it makes the belly expand and contract) or diaphragmatic breathing (because it uses the respiratory diaphragm) or even transverse abdominal breathing (because it uses the respiratory diaphragm) or some combination of all three of these.
Nauli kriya is another exercise that makes use of the respiratory diaphragm. (You suck it upwards so that the belly moves inwards. This is done while holding a full exhale.)
One interesting thing to note about the diaphragm is that it doesn't just sit between the organs, it is attached to them. It is attached via connective tissue to the bottom of the heart and lungs and it also supports, via connective tissue or direct connection, the liver, stomach, spleen and kidneys.
And, as mentioned, it also connects via connective tissue to the transverse abdominus, psoas and quadratus lumborum.
By being able to consciously activate and relax the diaphragm (so that you can move it up and down at will) you can lift and lower your internal organs and help to pull (and then relax that pull) on the psoas major and the quadratus lumborum.
With a very basic understanding of the diaphragm, you can focus on feeling your diaphragm while doing belly breathing or even while going to the bathroom. You can look for a "pressing down" feeling while in contracts and while it pushes your belly forwards.
However, this ignores the fact that the diaphragm attaches to the lower part of the ribcage.
When the diaphragm contracts downwards it presses down on the organs of the belly and causes the abdominal organs to press the belly forwards. However, at the same time it also pulls upwards on the ribs. (Dropping through the atmosphere, a parachute presses down on the air it is travelling through. However, to the person it is supporting, it feels like it is pulling upwards. A kite flyer pulls down on a kite but at the same time the wind pulls the kite upwards so that the flier feels an upwards pull.)
You can learn to feel your respiratory diaphragm by feeling its upward pull on your ribs.
Where on your ribs? The outer tips of ribs 12 and 11, and from there along an upwards sweeping curve that follows your ribcage around to the front of your body and along the "costal arch" that frames your abdominal muscles.
This is a rough guide, but from there you can move your awareness around and look for "pulling" sensations that you can then recognize as the fibers of your diaphragm pulling upwards.
Just because the diaphragm has one name, the "diaphragm" or "the respiratory diaphragm" doesn't mean that it is a single muscle that can't be broken down into parts. It may actually be made up of individual fascicles or muscle units, each of which attaches to individual ribs (as well as to the sides of the lumbar spine.)
As you develop awareness and control of your respiratory diaphragm, what you may find is that you can activate all fascicles of the diaphragm (at least those that attach to the ribs) so that you can increase your ability to breath deeper while using your diaphragm.
One of the ways that I learned to better control my diaphragm is via the kidneys. This was after first learning to belly breathe.
The assumption is that the diaphragm, kidneys and 12th rib are all connected. By lifting and expanding the back of your kidneys you are actually contracting the fibers of the diaphragm that support the kidneys and attach to the 12th rib.
Of course to do this it helps if you have a rough idea of where your kidneys are, particularly the top of the kidneys and the back. The best "general" description I can think of is that they are in the area of your lower back, but upwards from there so that they are half in and half out of the bottom of your ribcage.
If you put your fists into your lower back and then moved them half a fist up, that would be the approximate height of your kidneys, though the right kidney is usually lower than the left (because of the liver.)
The kidneys are in front of the 12th ribs. To activate the fibers of the respiratory diaphragm that attach to this rib focus on expanding the back of your kidneys (so that your bottom ribs feel like they are expanding both backwards and upwards at the same time.... you could also try expanding them out to the side also while inhaling.) Relax this feeling when you exhale.
Once you've got this feeling (and it may take some practice) you can then work at pulling up on ribs 11, 10, 9 and 8 while inhaling. Relax while exhaling.
The feeling you may get (and this is subtle) is that you are "flaring" the bottom of your ribcage when you do this action.
For myself what I find is that if I try to focus on just pushing down on the diaphragm, my inhales aren't always that deep. However, when I simultaneously focus on pulling up on these ribs (only slightly) while at the same time pushing down with the respiratory diaphragm, my inhales can be longer, as if I am activating more of the respiratory diaphragm.
Note that if you are already used to costal breathing (using the intercostals) and also if you are used to lifting your entire back ribcage using levator costalis, the feeling when activating the diaphragm in this way is quite a bit different. When using the levator costalis the feeling is of the entire back of the ribcage lifting and expanding. When using the intercostals the feeling is of the entire front and sides of the ribcage expanding... while inhaling.
The feeling when using just the respiratory diaphragm is different.
The interesting thing is that when using this method, you may find that you don't have to "deliberately" exhale. You can simply relax, and either the weight of the ribs, or the elasticity of the abdominal wall and ribcage helps to reduce the volume of the lungs and cause an exhale.
As a yoga teacher, I'm constantly exploring new exercises, new ways of doing yoga poses.
There is no single "right way" of doing a yoga pose. Instead, there are options. And the better you are at "feeling" your body, the better you can get at choosing the right option for your body as it is now.
For any technique, the point of practice is to learn feel it and to control it, so that it can be used without thinking about how to use it.
And that is more or less the approach taken in all of my ebooks and videos. They help you to feel your body and control it so that you can work towards using it effectively in anything that you do.