Modify Bezier direction handles in the Graph Editor

In the Graph Editor, keyframes that use Bezier interpolation have direction handles attached to them. You can retract, extend, or rotate the direction handles to fine-tune the Bezier interpolation curve in a value graph. You can retract or extend the direction handles to fine-tune the curve in a speed graph.

By default, when you retract or extend a direction handle, the opposite handle on the keyframe moves with it. Splitting direction handles makes the two direction handles attached to a keyframe behave independently.

To retract or extend direction handles, drag the direction handle toward or away from the center of its keyframe with the Selection tool.
To split direction handles, Alt-drag (Windows) or Option-drag (Mac OS) a keyframe with the Selection tool. You can also Alt-drag (Windows) or Option-drag (Mac OS) outside a keyframe to draw new handles, whether or not handles already exist.
To manipulate the direction handles of two neighboring keyframes simultaneously, drag the value graph segment between the keyframes.
Extending a Bezier direction handle in the speed graph