This is how all starting animators learn their craft adhering to the "12" without why the principles are the put in place then it has no relevance to a starting animator, I wanted to go over them again and explain why they are important. All of them are designed to create "the illusion of life" by following the physics of the world around us because without the idea of mass and weight properly expressed it is not "believable" to the audience.
Squash and stretch
Applies to muscle contracting and expanding, the force of pressure exerted or applied to anything that is more than the mass will push the object out of the normal shape.
The train of thought of a character shown through the face to the start of an action, the start of an action that requires the movement of the whole body to move in the action, the non anticipation to an action that is created can be used for comedy i.e. the banana peel
To show the personality of the character involved, how they react to the world around them. To tell a story, by how the character is shown in their environment or in angle portrayed or how small or big they seem in the world.
Straight ahead action and pose to pose
Both of these are important, whether you need to have control of the whole thing like when putting in overlap or micro expressions through Straight Ahead or where the story is going or who your character is, through the Key Frames with Pose to Pose.
Follow through and overlapping action
Everything does not stop or start at the same time due to the gravity, mass and weight of every single object, even when attached to something it will need to have the mass and weight of the starting object and its own to start moving and stop usually after the starting object just because it has the initial movement that has to overlap unless it is not affected by the start object i.e. very heavy.
Slow in and slow out
Shows weight of an object, it needs force exerted to start and force exerted to stop the movement (any movement has an equal and opposite reaction).
All things alive in this world move in arcs as it is more energy efficient to do so and the way that joints work as a pivot or as a hinge and muscle works as a "pull" and "stretch" for each action resulting in the natural "swing" of any movement. Also for anything that is thrown or is jumping through the air a natural "arc" is seen as the force of gravity is applied at start point, the apex and as the object returns to earth.
The body either an animal or a human is always moving several steps ahead of the conscious mind as it balances, breathes, muscles twitching, eyes tracking and ears swivelling to hear noises. A human trying to keep track of things they have to do that day or nodding to music while doing a task, these things make the character seem more "human" and therefore more worthy of our "time".
As the mass and weight of the object make it probable to portrayal through "effective" timing, how fast the object moves is dependant of the area (mass) as well as the weight of the object, the centre of gravity of the object i.e. is it closer to the ground to avoid gravity increasing the weight placed upon it (like a baby), does it live in something like water that negates normal gravity unless in the pressure of the deeper areas of the ocean.
Whether it needs to think about an action before it does it compared to an instinctual action, a person that is going to plan and execute the murder of someone close v.s. shielding the face from an oncoming object.
Also whether the character has done something before, several times or "could do it their sleep" as then the action is stored close to hand and imprinted on the memory of the muscles that aid in the motion (as in everyone still remembers how to ride a bicycle but when doing a new task seem clumsy and adept at it).
Is seen in most sports as you see the person hold the ball in the air before they throw it or the swing looks over exaggerated to make you notice what is happening when the people only look like little critters on your tv screen.
Follow through of the action also needs to be exaggerated as the audience can know that it is the end of the action before the slow into moving hold.
Also is useful in the acting of the character, whether the character wants sympathy by "hamming it up" or whether the character is a powerful the character will move much less with all the key poses being important and strong.
Through the silhouette of your poses it should tell you everything that your character is doing and thinking, if the pose is obscured or you cannot see the hands clearly and also you cannot tell the character weight or whether they might be lifting anything there is no point in the animation without communicating clearly to the audience "when","what" and "why" they are empathising with the character, they tend to get distracted and your time and your awesome animation is lost in the background.
Why is twinning such a big no, no? It is because it is unnatural to humans and animals to have both sides matching. We are always offset to balance our centre gravity and that's why we have "line of action" and our poses and that of animals are always interesting to see. We like to see thing that have appeal if they are acting unnatural, through timing or the posing we aren't drawn to it, the idea of the "uncanny valley" something that looks or acts human but through the actions of it will trigger repulsion.