The diaphragm is the primary breathing muscle and this exercise targets the diaphragm in a special way to strengthen this muscle. During the exercise, as pointed out earlier, you may notice the left side of the diaphragm working harder than the right side.

According to online MedicalNet: “The right hemidiaphragm is protected by the liver and is stronger than the left. The left hemidiaphragm is more often subject to rupture and hernia than the right. This may also reflect weaknesses at the points of embryologic fusion of the left hemidiaphragm.” The two sides of the diaphragm work together in coordinated, complicated ways and can even function independently of each other as seen in the fact that if one side is paralyzed due to injury the other side can continue to function normally. But the right side of the diaphragm must be considered dominant for breathing, and while the exercise targets both sides, the right side will always be dominant for healthy individuals.

As the diaphragm weakens with aging or inadequate exercise, it begins to recruit muscles in the middle core and lower core to assist with breathing. It also spreads up to the chest wall and it actually makes it harder for the lungs to inflate, and worse yet, it puts mind-slowing stress on the beating heart. The key to preventing extra tensing in adjacent muscle systems during quiet breathing is to isolate and target the diaphragm for strengthening. By forcing the diaphragm to work harder to move air, the diaphragm gets stronger on both sides and will naturally stop recruiting nearby muscles for help.

It is now easy to believe that all our discussions of intelligence need to factor in the physical condition of the diaphragm, because when it is weak it leads to excess tension in the upper body that depresses mental function. When the diaphragm is strong it promotes the highest possible level of mental performance. This is especially noticeable in our verbal skills: speech, singing, silent reading, thinking, writing, and even math calculating.

The exercise relies to some extent on a breathing position designed a long time ago to make breathing easier for those with difficult breathing. It was a widespread hospital practice. My research shows this position isolates and targets the diaphragm for improvement but it must be done a certain way. 

Pure Diaphragm Breathing

1. Lie on on your back on a comfortable surface with a bed pillow or two under your knees which elevates the knees. This relaxes the thighs which is an important condition of the exercise. There can be a pillow under your head. The head must face straight ahead. Tilting the head a little right or left tightens the breathing muscles from neck to core. Shoes off. Quiet atmosphere. 
2. Breathe this way as naturally as you can for 20-30 minutes. Do not force the breathing. Stop if this becomes necessary. Pay attention to your breathing.
3. The exercise can be done daily for a few weeks until no more improvement is possible and then it can occasionally be done to maintain optimal performance. It should not take long for you to start experiencing faster and higher quality reading along with many other rewarding upgrades in cognitive function.