Trial and error.  Focus it on different parts of the body and see what happens.  There are surgeons, too, as well as hunters, so they know the basics of biology.  At some point in the past a master might have focused life energy onto different organs to see what happens.  Heart? faster heart rate.  The lungs?  Recover faster after getting winded.  The stomach? Faster digestion.  The intestines? Food seems more nutritious.  The Liver? Recover from alcohol and some toxins faster.  The brain?  I feel smarter.  Muscles? I'm stronger.