Final Cut Pro X: Retiming clips overview
You can adjust a clip’s speed settings to create fast-motion or slow-motion effects. You can also reverse a clip, rewind a segment of a clip, apply variable speed effects (also called speed ramping) to a clip selection, and create instant replays and jump cuts. By default, Final Cut Pro maintains the audio pitch of any speed adjustment, but you can turn off this feature to accentuate the speed effect.
You can’t apply speed changes to still images, generators, titles, and themes in Final Cut Pro.