Step 2: Fine-Tune the Timing

Although you can spot-apply effects via the Timeline crop markers, you can further control the timing with the Effect In and Effect Out sliders. Say you want to slowly raise the exposure until the bright parts blow out, then pause the effect to heighten the suspense. Select your target clip in the Timeline and pick Exposure Adjust from the Quartz Composer subset of Video FX. Use the lower slider (EV: Exposure Value) to set the max amount of exposure tweak, then use the Effect In slider to specify the point in your selected footage at which the effect maxes out; move it to the right for a slow buildup, or to the left to have the effect start immediately. Now use the Effect Out slider to specify how quickly the effect fades-the associated value is the number of seconds from the end of your clip or selection that the effect will disappear

With Effect In and Effect Out both set to 0, the effect lasts the entire length of the clip.