From animation, a secondary action is an additional action that an animator might depict that adds additional realism to a scene. For instance, if a person is walking, the walking would be the primary action. The bouncing of their ponytail, or the flower on their hat would be the secondary action. For example, in Hayao Miazaki’s movie Spirited Away, there is an animation of the character Chihiro putting on her shoes. As she runs away, she quickly taps her left shoe to seat it better on her foot.
From the perspective of the animator, the double tap is a completely unnecessary action, but it adds significant realism and depth to the viewers experience of Chihiro as a real little girl in the bizarre fantasy landscape.