|Pull up and pull down functions are used when working with film that has been transferred to video for audio post-production. These two functions deliberately "miscalibrate" the sample rate clock of your audio, allowing you to compensate for the speed change that occurs when transferring film to video.
The issue here is that film runs at a rate of 24 frames per second (fps), while video runs at a rate of 29.97 fps. Film is usually transferred to video using a process called 'telecining' in which every 24 frames of video are mapped to 30 frames of video. Because video is actually playing back at 29.97 rather than 30 fps, the picture is running 0.1% slower than it does on film. The result is that when the video mix is transferred back to film it will slip one frame for every 1000 that goes by.
To solve this problem, post production engineers use the pull-down function to match the sample rate to the speed of the video (i.e. 47952 Hz instead of 48 kHz). After all of the production work is finished and the audio is ready to be transferred back to film, the engineers pull the sample rate back up to 48000 to insure that it will remain in perfect sync with the picture.
In summary: If working with film that has been transferred to video, you will need to pull-down the sample rate in order to be in sync when recording editing and mixing to video. By doing this you insure everything will remain in sync while working with your video. If you are going to create a final mix that needs to be transferred back to film from video, you would at this time pull back up to thetrue sample rate. Remember, If you do not pull the sample rate back up when transferring back to film, you will be out of sync one frame for every thousand that go by.