The woman's hand exerts a force that can break a board. But what about the force that hurts her hand? -- That's the reaction force, equal and opposite to the force she exerts. The faster she moves her hand, the more boards she can break, but also the more damage she can do to herself. -- To find out how she learned to break boards, visit Kung Fu Science. -- For examples of reaction forces, see Newton's Third Law of Motion from The Physics Classroom.