Some effects in the Video Animation Editor include fade handles, which allow you to adjust how long it takes for an effect to fade in or out. For example, you can use fade handles to adjust how long it takes for an opacity effect to fade in, how long it takes to fade up from black, or how long it takes to fade to black.
You can also move keyframes up or down to create a curve for the effect parameter and change the curve shape (interpolation) between keyframes.
Creating a fade or an effect curve smooths the transition so that the effect changes appear more natural when the clip plays.
1. Click the disclosure button to expand the effect in the Video Animation Editor.
Note: If an effect doesn’t have a disclosure button, it doesn’t have fade handles.
2. Drag the fade handle to the point in the clip where you want the fade to begin or end.
Fade handles from the beginning of a clip create a fade-in, while fade handles at the end of a clip create a fade-out.
Click the disclosure button to expand the effect in the Video Animation Editor, and do any of the following:
Note: If an effect doesn’t have a disclosure button, you can’t move keyframes up or down.
Tip: To add a keyframe and change the effect’s parameter value at the same time, Option-click while dragging the effect control up or down.
Tip: To adjust values with greater precision, hold down the Command key while dragging the keyframe up or down.
Do one of the following:
Command-Drag the line horizontally between the keyframes to create a curve.
Control-click the line between keyframes, and choose a curve option from the shortcut menu.
Note: You can change the curve shape between keyframes for video effects only. For audio volume only, you can change the curve shape for fades created using fade handles, but not keyframes.
from: support.appple.com/kb/PH12629