Ok, let me try to explain with an example.
- Create a cube
- Set any key at frame 0, 5 and 10. (doesnt matter what)
- Make the key at frame 5 a 'breakdown' key
- Goto the graph editor.
- Select the key at frame 10.
- Move it along in the graph editor and you will see the key at frame 5 will move along in relation with it. Thus keeping the animation intact. Think of it as a kind of scale. Although its got more uses than that.
It creates a relationship with the keys either side.
If you move keys without any breakdowns, then it will move just those keys on its own.
I sometimes use this method to scale small sections of my animations.