This is probably the worst of all motivators. If people do or don't do something because they fear a consequence, their actual behavior is unpredictable, because it may lead to other undesirable, destructive or anti-social behavior. Teachers should never threaten students as a way of motivating them to learn something. Power is a strong motivator, but has little application in most classrooms. If a person is motivated by power, it means that the person is engaging in some behavior because he wants to cause others to have to choose among alternatives that he sets.
Teachers have power.
They can cause students to have to choose from among alternatives that they set. There is a difference, however, between having power, and using it effectively. If you do something I want, then this will happen. The key in using power is to set conditions so that the desired alternative is chosen. There is a difference between using power effectively and using it ineffectively.