How does this work?

[Grassy chunks]

So, recently, i checked out the old and famous Trinity College wad (the one from 1994) and was startled by seeing the short movie that played in its "theatre" of a cyberdemon killing the marine. But, when i saw how all of this worked in slade3 i found nothing sort of hackery-ish... All the animation frame were named as MOVIE with the first frame being named as SLADRIP1.

 

So... do y'all know how this works?? As i couldin't find any info on how to make a longer animation with more frames than originally supported by vanilla doom. (Also seen in tnt evolution's map 30s animated skull switch)

https://doomwiki.org/wiki/The_Unholy_Trinity

 

[Gothic]

By adding more textures in between vanilla animated sequences (in this case SLADRIP1-3), you can make longer animations.