The vertical bobbing motion of the pigtail is be referred to as bounce. Bounce is made possible by a positive spring buffer. A bouncing cycle consists of the pigtail baseline being (partly) stretched by a vertical acceleration, and curled up again. This is a simple example of the classic mass-spring interaction, where potential energy is converted to kinetic energy and vice versa. In this case, the pigtail acts as both the spring and the mass.