Knife play is about physical or mental stimulation to danger (or fear). Knife are about being weapons and having a knife as part of a sexual encounter add danger to it. Knifes can be used as a temperature sensation. Being tied up and unable to protect yourself. The sense of something dangerous being dragged across your body can be exciting to some. A knife also usual feels cold when it is at room temperature. Your body is about 90 deg and the knife is about 70 degress so it create a sensation of coldness when it touches your skin. Knifes can also be used to peel off wax after wax play. The wax is a sensation of heat and the knife gives a sensation of coldness.