Your anatomy is separated into two distinctive types, Yin or soft, and Yang or hard.
The Yang or hard part of the fist is used to hit Yin/soft parts of the anatomy. You should never hit other Yang parts of the anatomy with the Yang part of the fist.
The Yin or soft part of the fist is used to hit the Yang/hard parts of the anatomy. The Yin part will be less effective if hitting other Yin parts of the anatomy.
The forearm is divided into the underside Yin/soft side and the top or Yang/hard side.