The Principle of Arcs in animation is based of the fact that most actions in real life occur in arcs as gravity causes objects in motion to arc between the start and end points.
Adding arcs to movements in animations will make the movements more fluid and realistic as can be seen in the image below from Dante Rinaldi’s posts on the animation principles
The shallower the arc the faster the object’s movement will appear the more pronounced the arc, the slower the object will appear to move.
This can be seen in this GIF of Toothless from Dreamworks’ How to Train Your Dragon where Toothless seems to slow down after coming through the clouds as the arc gets more pronounced.

Dreamworks – How to Train Your Dragon (2010)
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