"I've got it." At the moment John Acme vomited, Yusuf Zuck finally saw what was wrong; he had found the cause of the illness.
The flow of blood in the human body is always regular, maintaining a constant pattern until death.
However, when John Acme vomited, the blood in a certain spot above his stomach suddenly began to flow backward. It seemed as if something was blocking the blood when it reached that spot, causing it to flow back in the same manner.
This reversal was akin to what's mentioned in martial arts novels about 'True Qi' flowing in reverse. When the blood flowed back, the sudden compression triggered a reflexive vomiting response in John Acme.
Yusuf Zuck was no doctor, but he was certain that it was the blood flowing back that caused John Acme to vomit.