I bet almost every animator has practiced the follow through and overlapping action principle with tails, bunny ears and/or cloth. These exercises are actually relatively easy and fun and might misguide us into thinking that this is one of the easier principles – but it’s not. Those bits that dangle behind your character and can be added in your last pass of animation are not the core of this golden animation rule!
Follow through and overlapping action applies to the whole body – all the body parts in constantly changing interaction. It’s usually the hips, the shoulders, the head and the wrist that lead a motion (the initiator) at different times and whatever is attached (the effector) follows and comes to a delayed stop when the initiator already came to a hold. Which parts follow which other parts can even change within one motion. These are highly complex processes and the key to making your animation physically believable. The body parts take turns being ball and tail.
So, follow through and overlapping action have to be well planned and are more than just an extra. You can add clothing, tails and ears in the last pass but you should know what body parts drag, pull, push, follow and swing when and how much as early as possible. On top of that the follow through parts all have different weights or at times might be stiffened by muscle tension.
Those exercises that we all had to do at some point (bouncing ball, ball with a tail etc.) and that we might see as easy basics are present in every animation and go far beyond whatever helped us to understand the principle. So if you got the bouncing tail right, go animate somebody climbing a tree and look at the body as dozens of tails all influencing each other and a seemingly easy principle just got much harder to carry out.