The manner of breathing is either hard or soft.

Simplest answer: it is the way I was taught to practice the Isshinryu Seiunchin Kata. For my first 25 years it was my favorite kata practice. Then I got to the point all of them were my favorite kata.

 I know longer know how I was originally shown the kata. Almost immediately thereafter Mr. Lewis formed a yellow belt team, to perform Seiunchin for a demonstration. We were drilled in a specific performance incorporating hard breathing and normal breathing. Both Slow and Fast movements.

Thereafter that is the way I practiced that kata.

I know others did it differently. But in those days much was not discussed. You did what you were shown. And in those days applications of kata movement were not studied, it was just soldier on.

 Hard movements, intense breathing, Fast movements, normal breathing, both within the same kata practice.

This was years before I realized there was a difference between Japanese karate practices and Okinawan ones.

I was not around others in Isshinryu, just kept my practice up, and then this is how I taught it.

Years later studying under another instructor who trained alongside my senior instructor on Okinawa, I asked him about the breathing. He explained the instructor may have exaggerated the breathing so everyone could hear him.

Again years later, another senior instructor sharing kata practices from those days, did it the same was I was trained, stating that he was shown to practice it this way as a Chinkuchi training practice. 

I do not have the ultimate answer, it is just what I do to this day.

This is one of my students back in 1989. The video record does not capture the breathing, but it is there on the slow movements, as I taught him.

https://www.youtube.com/watch?v=Oq0KS5EaBwo

In time I came to understand more about the potential of breathing.

How inhalation during a technique could cause you to draw into the target.

What the difference was between how the definition of a technique mattered and that exhalation within a technique could add power to your technique and then how inhalation between techniques would also add to the use of techniques. 

Also the reverse also was possible. Thus as you controlled which version you were using made you did more un-read-able by an opponent.

I then worked out a way for  black belts to train in each method, for training is necessary to utilize breath.

To breathe is to exist.