The breakdowns are poses that go inbetween the keys when there is a very big action such as a character sweeping their arm across in front of them. The first key shows the arm across their chest and the next one shows it on the other side of the body, wide open. What goes in between this? What is the position of the arm and hand? Try this action yourself. As you pull your arm across, notice the position it makes in the middle. Stop it there and take a look at it. Now try it again, but change the path of action so your arm goes down first and then up to the other side. The middle pose is different from the fist try. Try it again now, but this time arc your arm up in the middle. What if you did a little flip of the wrist in the middle?
This is where the breakdown answers the question.
If the action is something elaborate like the flip of the wrist, then the animator is really responsible for drawing this pose. For the upward or downward path of action, the animator could simply indicate the path of action with a line and a small ball or rough drawing to indicate the position of the hand.
This is the purest definition of a breakdown.
When youre animating, you draw all the main key poses. These are called primary keys. For some actions, such as anticipation, overlapping action, or recovery you might need to draw what are called secondary keys. Now theyre all really just keys but when you sit down to draw a scene you may go back in and add some of these extra keys just to punch up the action some more. Some animators do them in sequence: 1, 2, 3, 4, 5, 6, while others might go back and add some in like this:1, 2, 3, 3a, 4, 4a, 5, 6, 6a. 3a might be the anticipation key, 4a the overlapping action key and 6a the final recovery key.
While its very rare that this happens, an animator might leave these up to the assistant whom they feel is ready for the challenge. This is usually an assistant who is climbing the ladder towards eventually becoming an animator themselves. The animator is then apprenticing the assistant and giving them a larger share of the responsibility on the scene.
The important thing about primary keys, secondary keys, or breakdowns is that they convey the information about the action taking place in an appropriate manner.