Open the retime controls
Select a compatible clip and choose Clip → Retime (⌘R). Constant speed, reverse, holds, and speed changes affect the relationship between sequence time and source time.
Changing speed can change duration. Check the clip’s position and surrounding material after applying a retime rather than assuming every edit remains in place.
Choose picture treatment
Frame sampling and optical flow produce different pictures. Optical flow estimates motion between frames; complicated movement, occlusion, fine detail, or fast crossings can reveal artifacts. It is not a guarantee of clean slow motion from any recording.
Review the actual moving result, especially where speed changes and where the subject crosses the background. Inspect original-media output for final delivery.
Listen and check boundaries
Retiming also requires attention to sound and source limits. Listen for undesirable audio behavior and inspect linked components. Review the first and last used source pictures and check that the requested range stays within the source.
Keep an unretimed version when trying an expressive speed effect so timing and image quality can be compared directly.