In one type of breathing exercise you pull your lower belly inwards just prior to inhaling into the upper belly and the ribcage. You could actually think of this as uddiyana bandha.
The uddiyana part is the pulling in of the lower belly. This then gives more room for the upper belly to lengthen. Since the upper belly can lengthen more this means the ribcage has more room to move upwards. And this gives the diaphragm room to push downwards so that the overall feeling can be of lengthening or lifting the ribcage upwards. Unless you have a very flexible belly (and even then) pulling the belly inwards may make breathing using your diaphragm easier, particularly if you are trying to keep your spine straight at the same time.
Of course to feel this affect it helps if you know how to breath diaphragmatically. You can first practice breathing diaphragmatically with your spine bent forwards and belly relaxed (so that you diaphragm can easily press outwards on the belly). Then you can practice sitting up slightly taller while using the same technique. Then sit taller still.
You'll probably notice that the taller you sit the more challenging it is to breathe into your belly. That's because as you sit (or stand) taller, you straighten your spine and that adds tension to the belly. And so that diaphragm has to work harder to push the belly forwards.
However, if you pull the lower belly inwards, doing uddiyana bandha, then the diaphragm can more easily push outwards on the upper belly (via the abdominal organs.) Breathing diaphragmatically is then a little bit easier.
To help isolate your lower belly, agni sara may help.
Another way to practice diaphragmatic breathing is to straighten the spine as you inhale. This involves lifting the chest which in turn involves lengthening (and adding tension to) the front of the belly. Again this can make it difficult for the diaphragm to press downwards because it is working against a belly that is gradually being made more taught as you sit or stand up taller.
Here again, pulling the lower belly inwards makes it easier for the diaphragm to work. Generally when using this breathing method I like to pull the lower belly inwards first, then inhale into the upper belly and ribcage as I straighten my spine. The feeling can be like the diaphragm, as it contracts downwards, is helping to push the front of the ribcage (or lift it) upwards.
(While exhaling you can relax your belly and allow your spine to bend forwards.)
So what about mula bandha and uddiyana bandha together? Mula bandha can be thought of as "engaging" the pelvic floor. You can focus on just activating the PC muscle, or on gradually activating the entire pelvic floor, even including the piriformis if you choose (and even the psoas.)
Mula bandha in general can be activated by pulling your tailbone towards the pubic bone so that you get a line of tension between the tailbone and pubic bone. To add more tension (so that you can feel it more easier) you can also pull your anus towards you pubic bone and for even more "feeling" pull your perineum (just in front of the anus) forwards too. (This is from a male perspective. The pulling forwards just in front of the anus may be different for women. You may have to focus on pulling forwards from a point slightly higher within your body, and perhaps slightly back or slightly forwards.)
If you start with mula bandha you may find that it is relatively easy or even natural to pull your lower belly inwards also. If you imagine you could reach around from behind and place your hand between your legs with your finger tips above your pubic bone, then the muscular tension would be like the fingers tips of the hand drawing downwards and back towards the heel of the hand.
How high up from the pubic bone do you pull the lower belly in? Try a fingers width directly above the pubic bone, and then a little higher. Generally you might think of the "lower" belly as the part of the belly a handswidth above the pubic bone (that's a handswidth with the fingers together!)
Now when doing this "uddiyana bandha breathing method it's easy to focus on the lower belly and upper belly. You may also find it relatively easy to feel and lift your chest.
It can be easy to forget your diaphragm especially when the belly and chest are expanding and lifting while the diaphragm is supposedly moving downwards.
To bring at least some partial awareness to the diaphragm while breathing, and also bring awareness to the back of your body you can focus on lifting the backs of your kidneys when you inhale. The connective tissue here attaches both to the kidneys and the diaphragm and also has connections to the front of the psoas muscle. And so as you pull your lower belly in you can focus on drawing a line back and up behind the back of your kidneys. As you pull your lower belly in, continue this pull to the back of the kidneys and pull them upwards aswell. As you do so you may find it helpful to pull in and up on the belly both just above and just below the height of the belly button.
For extra mindfulness practice you can also focus on feeling the five vertebrae of your lumbar spine. As you inhale gradually straighten your lumbar spine. Since you are already focused on feeling your kidneys, it should be relatively easy to expand your awareness to also include your lumbar spine.
However if this isn't possible then focus on each of these points in isolation. When you get comfortable with one action, work on another action and then add them together.
At any point you find yourself getting overwhelmed or you have to stop and think about what you are trying to do then make what you are doing simpler. Focus on a single element again (say pulling your lower belly in) until you can do it relatively easy, and then when you are ready increase the complexity of the exercise.
Then when you can do the breathing exercise easily, then see if you can do the same breathing exercise while doing yoga poses. You can then judge whether uddiyana bandha helps what you are doing or hinders it.
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