Some bacteria use type IV pili to move along surfaces within a biofilm. (Biofilms are to viscous for bacteria to swim through with flagella.) It starts with a bacterium extending and anchoring its pili. As the pili contract, the pili become stretched or taut. As the anchoring pilus detaches, the taut pili "slingshot" the bacterium in the opposite direction. This motion alternates with the twitching motility also caused by type IV pili and enables a more rapid motion and direction change than with the twitching motility because the rapid slingshotting motion reduces the viscosity of the surrounding biofilm.