Alright, so I sent this message to Violet on June 19th in hopes that he would update his article. I might as well post this here in the meanwhile because "background triggers" have more uses than just in the background and the effects apply to all types of scenery events (see my Fortress of Fate Assault level). So here's the original message slightly edited:

Let me see if I can explain this. For starters, a level in JJ2 can be divided into 4-tile wide segments starting from the top left. Information on what's in each segment is saved, which is probably done to save memory and reduce file sizes. Anyway, take a look at this.



This scenerio would work perfectly fine, assuming an animation for ButtStomp Scenery was used. Identical sets of 4 tiles are needed, and flipped tiles count as different tiles. Keep in mind that each set of tiles must fit into a 4x1 segment in JCS.

In the same layer, the set of tiles furthest up and/or left (even with no scenery events) affects all other identical sets of tiles that have no events on the animating tiles (just so you know, you can use any event other than scenery events on tiles that aren't animations with no ill effects). Higher layers dominate lower layers, so if an identical set of tiles was in a foreground layer, it would prevent the set of tiles in the sprite layer with scenery events from affecting any other set. If you want to use scenery events in background layers, it doesn't matter which 4x1 segment you use.

Tell me if you have any questions since I probably didn't do the greatest job on explaining this.