The pogo effect consists of unstable, longitudinal (up and down) oscillations induced in a launch system, mainly due to fuel sloshing and engine vibration (see chuffing). These oscillations can lead to variations in the propellant flow rate, which in turn can cause the thrust from the affected engines to fluctuate several times a second. Periodic fluctuations in thrust occur in all rocket engines, but normally the mass of the rocket is sufficient to keep it from becoming noticeable or dangerous. However, when the thrust fluctuation approaches the resonant frequency of the rocket structure, the forces couple and the result is the sense that the rocket is surging backward and forward along its length several times a second. This motion resembles that of a pogo stick, hence the name.