Breakdowns are the key secret to good animation. They make the difference between good animation and great animation.
Breakdowns aren't quite keyframes, but aren't quite an inbetween either. I like to call them "Key inbetweens" sometimes, as they kind of are when you understand them.
Breakdowns are like special inbetweens you put in between keyframes to change/modify a motion. Almost always for the better. It makes motions feel more natural and flowing. Breakdowns are mostly used to:
1. Flow to a different "sub-pose"
2. Create overlapping motion
3. Add squash and stretch
4. Create flexibility in motion
I could go into more detail, but I think it'll be easier for everyone if you just look at the same sheet my animation lecturer gave to me way back when. Download it and view it here if you want.
You should also note that you don't have to use them, not everyone does. You also don't have to use them everywhere, GOOD animators know when and where to use them.
The pose can be a regular inbetween apart from a stray leg, hand, or hair, or a scarf. You don't have to apply a breakdown to the entire body/pose.
Here's a real quick animation of an arm lifting up off a surface, then landing back down on the surface:
See how boring and robotic it looks? This is because there is no breakdowns. Here is the exact same animation but with 2 breakdowns added. 1 exactly halfway between each keyframe:
Notice the huge difference? It has the exact same amout of frames, the exactly same timing and the keyframes are exactly the same as the above animation. The only difference is that 2 breakdowns have been added. The remaining 4 frames were inbetweened accordingly.
In the case of John's f+H, I'll be using one after every keyframe, bar the last one. Just like the example above, it really needs breakdowns to make it good.