The National Fire Protection Association (NFPA) started producing its codes and standards in the late 19th century, and NFPA continues its important work today. Over the years, NFPA produced a number of short videos that demonstrate how quickly fire can develop and spread. Some of these videos are posted on YouTube for anyone to view. Links to several of these videos are listed below, for your convenience.
This video appears to be an excerpt of the full length video. At about 2:45 on the video timeline, the narrator points out the "breathing" of the fire that occurs near the bottom of an open door. For the purposes of this video, there was no door leaf in the frame.
Swinging fire doors with builders hardware—as covered in Chapter 6 of NFPA 80, Standard for Fire Doors and Other Opening Protectives—are permitted to have not more than 3/4-inch clearance between the bottom of the doors and the floor surface directly below the doors when the doors are in the closed position.