According to Oregon State University, the asthenosphere is the part of the mantle that bends like plastic. This is because the asthenosphere consists of volcanic material that is both solid and flowing, giving it pliable characteristics.
An example of material that is both solid and flowing is toothpaste and the way that it squeezes from a tube. This is called plastic movement or plastic behavior and is similar to the convecting mantle within the asthenosphere. This plastic behavior means that the convecting mantle within the asthenosphere is a contributing factor to tectonic plate movement since the lithosphere above it moves with the asthenosphere.