The expressions “Pull up” and “Pull down” mean the re-calibration of the sampling clock, in order to compensate the change of speed. Both are based on the concept of synchronizing the audio change of speed with the video speed, because the film has been transferred to NTSC with a 0.1% variation, or the Pull down was performed. The same should be applied in the process to keep its synchronism.

¿When should Pull up or Pull down be used?

There are many ways of keeping the audio in post-production. The audio source and its final destination should both be carefully considered.
In some cases the audio requires the application of the Pull down. In others it should be used temporarily along with Pull down. Sometimes you can choose to perform the Pull down in a machine where digital-analog-digital conversion is used to record audio with the approved sampling rate; otherwise you can choose only Pull up or Pull down for the final audio managing.