Secondary action is an action that supports the main action. Let’s say someone is sadly, the main action that they are walking but the secondary action would likely be the individual slouching with their arms drooping to the side. To discover secondary actions, it can often be discovered through understanding of anatomy. let’s say a cheetah turning the main action would probably be the main body being at a sharp angle in relation to the ground, but the cheetah’s tail still follows the motion, adding to the sharpness of the turn.

https://www.schoolofmotion.com/blog/secondary-animation#:~:text=Secondary%20animation%20is%20any%20additional,%2C%20movement%2C%20or%20even%20sounds.

https://pixune.com/blog/secondary-actions-in-animation/