Overlapping is the transition of key frames in an animation - it breathes life into images. Take for example a picture of a boy throwing a ball. We start with a frame of him holding a ball, and jump into the frame wherein the ball is already on the ground. You can see that the situation we are trying to picture out seems lifeless. This is where we need overlapping.

The tricks in overlapping are usually those we see in videos that create little interest in us because it is such an essential part that our subconscious mind would think it only necessary for it to be there.
The perfect way to appreciate overlapping in animation is taking for example the antenna in the movie A Bug's Life. You will remember that the antenna did not look floppy at all but rather has realistic movement that instantly tells the mood of the characters.

The overlapping action is the tool animators use in creating life-like animations. You may observe that in some videos the movements of some parts of one character moves more quickly than the rest, these are also known as delayed parts. The right thing in doing this like a boss is simply going back to elementary science. You have to know the right momentum, and which among the parts has more inertia.

Inertia, as you probably well know, is the resistance in any object in motion or at rest. When we first try to move an object (leading force), there is a lagging motion (secondary force) because of inertia, but suddenly it moves because it can no longer resist to do so, hence the rest of the object moves in accordance to its own momentum. That is what creates life to animation. Animated videos seem so real because it sticks so much with what real life is about. This is what Donald Graham of Walt Disney studios called, €Animate forces, not forms.€

I suggest for you to incorporate each key parts of the character with one another, but make sure they don't clash in fighting for attention. As an animator, your timing has to be perfect. Every detail smooth and high in impact. It is key to know which are the key actions that must bring a reaction to other parts, such as hair tail, dress, etc.

Remember that the bottom line of using overlapping actions is to create smooth, fluid-like transitions in you animated videos. So overlapping actions have to observed in making even the simplest of all images such as the raising of an arm. Below is an example.

There must be a detailed movement first from the shoulders, to the arms, to the wrist and finally to the fingertips. Added in sequence, this creates a nice, fluid-like movement very similar to real-time. Keep track of the spacing of each frame. Once you've done that, it'll be easier on your part to dictate the movements of the succeeding frames.

You only have to €feel€ the weight of things you are animating.