Breakdowns are special keys that maintain proportional time relationships with neighboring keys.
In the first example, a normal key is inserted and a neigboring key is moved. In the second example, a breakdown key is inserted, and the neighboring key is moved, and this time the inserted key adjusts to accommodate the change in the shape of the curve. Note that the breakdown key is solid green.
When active (the default), this setting ensures that added keys affect the shape of the curve but do not change the neighboring tangent types.
Turn this option off if you do not want new keys to affect the shape of the curve.
In the first example, the Preserve Tangent Type setting is On (default), and a key is added to a curve, changing its shape slightly, while adjacent tangent are not changed. In the second example, the Preserve Tangent Type setting is Off, and a key is added to a curve but the curve shape does not change.