Which principle is typically used to indicate another action happening that supports the main one?

The principle that indicates another action happening to support the main one is secondary action. This principle is used to add depth and interest to the animation by introducing additional movements that complement and enhance the primary action. For instance, if a character is walking and swinging their arms, the natural swaying motion of their clothing or the bobbing of their head would be considered secondary actions. These elements don't distract from the main action; instead, they contribute to a more believable and engaging animation, providing a sense of realism and detail.

Other principles like anticipation focus on preparing the audience for an upcoming action, while follow-through refers to the continuation of motion after the main action has stopped. The arc principle deals with the way objects and characters move through fluid, curved pathways rather than straight lines. While these concepts are essential in animation, they do not specifically relate to the occurrence of additional actions that support a primary action the way secondary action does.