>>442296>sudden change of background …
Erm, I forgot to tell you that Flash is a multitude of softwares meshed together to make it "multiuse", whereafter, if you didn't knew already, Flash is a game engine aswell!!!
Hence you could use code into an apposite section, and generate any sort of "special effect" especially if your main use will be for art, instead of gamedev.
Flash uses ActionScript 2.0, and I'm pretty sure that background change was made out of strings of code, because otherwise I have literally no idea how he could've done that…
Guh, Besides you should try to see online, possibly in some old dead ass flash forum, or on Newgrounds if you find any relevant guide abt.
OR HECK IT why don't you get the Flash archiver and download a bunch of old animations and decompile them to see how they did said effects… if they were indeed code-based or drawn-based!
My biggest tip would be, if you can, decompile as many animations to see how they truly functioned behind the curtains!