The method used to
telecine films to
NTSC video. Film runs at 24 fps, but NTSC video runs a 30 fps (or 60 interlaced fields per second). To get these to numbers compatible, film frames are captured for 2 and 3 video fields alternating.
Film: 1 2 3 4 5 6 7 8
Video: 11222334445566677888
It means that NTSC DVDs run at the same speed as the original film, but it sometimes introduces an unpleasant jerking in slow panning shots.