[burfadel]

Most things are filmed in 23.976, and are telecined to 29.97 to match the TV standard. The TV standard of 29.97 for NTSC was originally to match the power frequency of the US, which is 60 Hz (2 fields of 29.97, aka interlaced = 59.94Hz). PAL is mostly 50Hz for the same reason, the power systems there are 50Hz! (there is a 60Hz PAL for countries that have a 60Hz power system).

For countries that have 50Hz PAL, the Film speed of 23.976 is sped-up by around 4 percent, which is not that perceptible when you watch it. Thats why the PAL versions of movies are shorter than NTSC versions, a 100 minute movie in NTSC is actually only 96 minutes in PAL (even though its the exact same content). For NTSC, 29.97/23.976 = 1.25 (it sounds wrong, but put it in a calculator you'll find its exact). So, what they do is have 5 frames in place of having 4 frames. The extra frame is made by mixing the top and bottom fields from two adjoining frames, and this is done twice not once which probably would make more sense in a way. The 4 frames are still present in the 5 frame group, but 2 frames are now mixed, so you have 3 original frames and 2 mixed frames (telecined). THis is why the term 3:2 pulldown is used on devices that remove the telecining via hardware. The downside to telecining is the mixed frames have fields which are from a different moment in time, as well as a new false time period created by having a new frame, which is why 29.97 shudders in movement which is particularly noticeable in horizontal pans.

Deinterlacing 29.97 without reoganising the mixed frames for compression is bad, as once it is deinterlaced it is much harder to get back to the original 23.976. Also, encoding at 29.97 is less efficient as you are not only encoding the same material but spread out over 5 not 4 frames, but the detail in 2 of those frames is actually different to the other 3 as you have mixed fields from different frames, which may have different moments of motion due to the different time of the original capture of the frames that are now mixed.

Taking all that in to consideration, inverse telecine (IVTC), aka 'Forced film' is definitely a worthwhile exercise on 29.97 material that was originally 23.976. I believe there is very little material that is actually filmed in 29.97. I'm not sure whether 'Forced Film' option in DGindex is a more suitable option than using an avisynth IVTC filter, maybe someone can clarify that. Setting 'Forced Film' in Dgindex though has the downside that it should only be selected for telecined material, which you just have to remember you selected that. Using an avisynth IVTC filter you can at least see it present (usually) and remember that it is set!