Adynamic stretch is
safe, because it involves gradual and controlled movements
safe, because it extends the joint beyond its range of motion
unsafe, because it uses the body's momentum to stretch the muscle
unsafe, because it requires quickly stretching the muscle and joint

A dynamic stretch is safe because it involves gradual and controlled movements. Dynamic stretching is the practice of movements before enacting them. For example, if you are weight lifting, dynamic stretching will practice the movements required to lift the weights without any load bearing. This warms up the muscles and makes sure that they are ready for the actual load bearing movement. The opposite of dynamic stretching is static stretching. Static stretching is the where the stretch is held for any length of time. 

How do we if it is a video and they know your face.