Well each part of the body has a score multiplier (some parts will take more damages than others depending on the mod rules)

For example on Lenshu3ng.tbm you will take no damage on the knee/elbow but the head will take many damages.
Its also based on the velocity of the part hitting the other (as in real life i want to say, if you don't move your punch fast, you will make no damages)

---EDIT---

Its clear in my head, but you will understand once you try it i guess