Final Cut Pro X: skimmer
The skimmer lets you preview clips in the timeline or the browser without affecting the playhead position. You use the skimmer to skim, or freely move over clips or projects to play them back at the speed of the pointer.
The skimmer appears as a red vertical line as you move the pointer over the area you’re skimming. If you have snapping turned on, the skimmer turns orange when it snaps to a position.
When skimming is turned on, you can skim to see what’s in other clips, while maintaining the playhead position in the timeline or browser. If both the playhead and skimmer are present in the same clip, the skimmer becomes the default position for playback or editing.
When skimming is turned off or the skimmer is not present in a clip, the playhead assumes the default position.