I learned that when doing yoga you need to keep the in breath and out breath the same langth. Then I learned from a different yogi to breath deep into your belly and breath out as much as you can using your abdominal muscles and squeeze your stomach in. Is there only one I should use or is the a mix or is one type for yoga and the other for meditation?
I can see we’re going to very quickly wiggle our way into a box, Pandora’s box!
Equanimity of breath, in terms of the duration of inhalation and exhalation serves a purpose. Breathing in to a count of 4 and out to a count of 8 (2 part breathing) serves a purpose. Breathing in using the Ujjayi breath (and out normally) serves a purpose. Breathing out using the Ujjayi breath (in normally) serves another purpose.
The second thing you describe sounds like one of the bandhas or locks. If that is the case, and I stress IF, then it too has a particular purpose.
Generally speaking, the breath does not get consciously moved into the belly. The belly may rise and fall. However the breath remains in the home of the breath which is the lungs. Ergo the breathing is between the diaphragm and the collarbones.
If you are asking about breathing for asana, and you are asking me, then the answer (from me) is to keep the breath smooth, observe it, and once taught, use the Ujjayi breath in the poses such that the breath is never choppy or disrupted.
I was surprised to read in a Yoga book that yoga is 90% breathing.
I was surprised to read in a yoga book that kapalabhati is advocated for beginners.
I’m surprised to learn yoga has room for competitions.
I’m surprised to learn mirrors, sweat, and rushing facilitate evolution.
Human beings. What to do with them???
Fortunately it was not Pandara’s box that got wiggled into…
As IA says there are so many views on breathing. Important for you is to follow the teachings of your teacher/yogi, he/she might have specific reasons why they do breathing in a certain way. For example my own teacher did a breathing exercise with me on a regular basis with great effects. To date I have no where in any of the many books on yoga which I have, came across this technique. Does it make it wrong? All I can say is she knew something that I needed and it was correct for me.
Every human being is different and so is every teacher. I think we should all follow what works best for us.