Keyframing

What is a Keyframe?
When we set a 'Keyframe' in FCP, which we set on a specific video frame either on the TL (Timeline), in the Viewer, or Canvas windows, we are 'marking' that video frame as the starting place for something to begin changing. When we set a second Keyframe, further down the line, we are marking that video frame as the place were we want the change to be complete. FCP will interpolate or generate all the frames needed between the 2 Keyframes so that the change will happen smoothly, over time.

What can be Keyframed?
Well, in FCP almost everything can be Keyframed. Some of the effects that we can Keyframe are: filters - color, size, opacity, rotate, mattes, masks, blurs, composites and change from color to Black and White. The only filter that can not be Keyframed is the,' de-interlace' filter. We can place any number of Keyframed filters on a clip and have them all running at the same time, all changing independently of each other. Very powerful stuff.

Keyframing
There are almost as many ways to apply Keyframes and work with them as there are things that can be Keyframed. We will start by doing our work in the Viewer window. We are going to change a video clip from color to black & white and then back to color again, over time.
I like to use the 'Sepia' filter which is found in the Image Control folder in the Effects browser. Drag and drop the Sepia filter from the Effects browser onto the clip in the TL. Double click on the clip in the TL to load the filter into the Viewer window and click on the Filters tab.
The first thing you will notice is that the Sepia filter default setting is set to 100 in the Amount box (Red Arrow). The Amount box controls the amount of filter that is applied to the clip. 100 being full filter effect, 0 being no filter effect. To the right of the Amount box you will first see the Keyframe button. Further to the right is the TL. In the TL you will see a horizontal green line. This is what I call the Keyframe line. You will notice that it is all the way at the top of the space as the filter amount is still set to 100. You can click on this Keyframe line and drag up or down to change the amount of the filter that is applied but this will effect the entire clip. The 'Red X' button under the word 'Nav' is the reset button and will return the filter to it's default settings.

(Quoted from: http://www.kenstone.net/fcp_homepage/basic_keyframing.html)