This is a door activated by alerting the caco behind a long tunnel concealed with a midtexture and impassable flag - as soon as the caco at the end of the tunnel spots the player it moves and teleports away, allowing the voodoo doll blocked by it to move along a current (which doesn't affect flying enemies such as the caco) and trigger any action.

Although it may seem unpractical, this allows you to trigger an action right after the player teleports somewhere, so for example a door opens as soon as the player presses a "Sx Teleport" or passes a "Wx Teleport" line, so that even if the player stops and remains in one stop after triggering the teleport, an action will happen - you can see it in action in the wad. The only problem is disposing of the cacos/lost souls/pain elementals - you can send them to a combat encounter or to a crusher, whatever you need

I don't think the player can reliably hit the voodoo doll or the caco, but you can make the tunnel even tighter(as long as the caco still sees you) to make it even less possible

someone could probably think of some other use for this, for reliably triggering an action when the player enters some large zone maybe

This of course works only in Boom and above, and sadly gzdoom is a bit fucky which forces you to tweak the box that bounds the caco a little to make it work in the chicken sourceport