What you want to do is add an effect to only a certain part of a clip, having the effect enter, and exit the clip, smooth and subtle. To do this, you need to get into
keyframes. When you click and drag an effect to the clip you want it to be on, a box will come up where you can 'tweak' the effect. On the bottom of the box that comes up, is where your keyframe work is done. What you do is, go ...