Need help with creating flashing lights

I was playing Community Chest 2 and saw a really cool lighting effect on map02 that looks like this:

I found it really cool so I opened the map in Doom Builder to see how it works but I am a bit puzzled. It appears that each sector that flashes has a tag but no effect. And the maps don't appear to have been made in Zdoom format so their is no scripting as far as I can see either:



Can anyone explain to me how that lighting effect was created?

It's probably some kind of transfer linedef....

Spleen said:

It's probably some kind of transfer linedef....

Is that a boom effect?

Take a look at the linedefs up there.