Good post and great tips.
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A large inspiration to the count of 3 expands the chest cavity and assists the ventricles of the heart to fill with blood (negative thoracic pressure), hence promoting circulation (a good thing during exercise). A rather shorter exhalation to the count of 2 more speedily eliminates the de-oxygenated air that needs to be wasted from the lungs quickly in order to refill them with oxygenated blood. This is the rational for inspiring deeply for the count of 3, and exhaling to the count of 2. In this light, to reverse the count (2 in; 3 out) makes no sense.

Shallow, rapid breathing, contrary to what another post claims, does not result in induced autonomic hyperventilation, as tidal volume for the lungs is never reached and CO2 is never cleared in excess under such conditions. On the other hand, rapid, intentional, DEEP breathing during rest can result in hyperventilation since CO2 is thereby blown off at an excess rate. Induced hyperventilation would most likely be more of a concern if the oxygen demand is small, such as while not exercising. Natural, physiologic hyperventilation (to a moderate degree) automatically occurs when one exercises. To intentionally induce a modest intentional hyperventilation in the anticipation of (or during) a burst of effort should not be a problem, and may in fact be assistive in surmounting the increased oxygen debt incurred by extra exertion.

The diaphragm is an inhale-only muscle. It pulls the bottom of the lungs downward which fills them. Contrary to the yoga links above, the diaphragm does not spread the bottom of the ribs–muscles can only contract, not expand. The intercostals can spread the bottom of the ribs because each lower rib is lifted by the muscle above it.

When the diaphragm comes down, the guts have to go somewhere so they go forward, which looks like the belly is breathing. That’s the inhale part.

Exhaling from the belly is very good for runners and all the time. Exhaling from the belly would not contract the diaphragm, but relax it. To exhale from the belly, the abdominal muscles must be contracted. These are just the abs, the same ones you exercise doing crunches etc. By forcing your belly inward, the guts have to go somewhere so they push the relaxed diaphragm upward. The diaphragm going up expells air from the lungs. This also helps the food move along the intestines

I have noticed that as I age (now 47), that my belly protrudes more than it did when I was young. This is not because of fat but I believe because my abdominal muscles are weaker. I believe using forceful exhalations using the abs helps the belly to come back to a younger state.

Forcefully exhaling from the belly is called “rhino breathing” by Miller in his book, “Programmed To Run”. He agrees forcing the air out means less CO2 in the lungs. He says the hemoglobin is 20 times more likely to take a CO2 molecule than an O2 molecule, so even a tiny bit more exhale has huge benefits.

One reason a person would narrow the mouth during exhale (blowing), is to increase the pressure in the lungs. This increased pressure would force more air across the lung barrier. This is kind of like a supercharger or turbocharger on a car, in comparison to a car that is “naturally aspirated.”

However, if the remaining air in the lungs had CO2 in it, you’d be forcing CO2 back into the body, which increases aging, lactic acid, and anaerobic wastes. Narrowing the mouth also makes it more difficult for the abs or intercostals to push air out of the lungs. Just as a supercharger takes horsepower away, so does supercharging your lungs.

There is no need to narrow your mouth. Leave the mouth naturally open and the nose too, for the most efficient full exhale of spent gasses.

The 3:2, 2:2, and 2:1 ratios are a good place to start. I usually begin my runs controlling my breath consciously, then transition into trusting that my body is wise enough to decide what is most natural in changing conditions. As my pace lengthens out mid-run, I often find myself using other ratios like 4:3, etc.

Happy exhaling!