Precise or Fast: which one to use
A video file stores a full picture only every second or two (a keyframe) and small changes in between. Fast mode copies the file as it is, so it must begin on a keyframe: the clip can start up to a couple of seconds before the point you chose, but quality is identical to your original and a 40-minute recording takes seconds. Precise mode rebuilds the section frame by frame, so it starts exactly where you asked. Use Precise for a clip you will post, Fast for cutting a long recording into something smaller to work on.