I don't know that anyone really understands it but I agree  that it allegedly happens. I understand that St Francis of Assisi  experienced wounds on his hand, feet, and his side on his death bed.  Can ones mind be so powerful that it can cause physical changes whether that be wounds or illnesses, I don't know.   Many in the medical profession say so, and that's why they say some people experience psychosomatic illness.....you think you're ill so you become that way.  You might check out a little book called "As a man Thinketh" by James Allen.  If you consider this BS, I apologize.