This is my take of the story after hearing many versions over the years, including the picture the Mod previously linked to.
The Chazon Ish was an Illuy a genius, who used his knowledge primarily for Torah.
However, he would have been at the top of his field had he chosen any branch of knowledge.
So when the brain doctor hesitated to operate, the Chazon Ish spoke to him and asked to be shown a diagram of the surgical site. (That is where the famous diagram comes in).
When seeing the diagram with the explanation of how the standard procedure of operating was done, the Chazon Ish reviewed the info with the diagram and suggested an alternate approach which impressed the surgeon who followed through with that method.
Not to compare my wife to the Chazon Ish.
Many years ago my wife needed some facial surgery that was to include some plastic reconstructive surgery too.
She asked the doctor about what it was going to look like afterwards.
My wife was flabbergasted that she had to offer that suggestion even though she has no medical knowledge, just lots of common sense.