Most effects are keyframeable. Find the frame where you want the effect to begin, use the back arrow key to move one frame back and apply the effect to the clip and click the “Toggle Annimation” button in the Effects Control Palette. This will set a keyframe of default values which is typically (but not always) no change. Move one frame forward and change the values of the effect. This will create another keyframe with new values. Go to the end of the part you want the effect applied to and click the “Add/remove keyframe” button to set a keyframe of identical value as the previous. Move one frame forward and change the value of the effect back to default, setting a 4th keyframe.