Actually easy to do but I had trouble figuring it out because of lack of good documentation.
First dig yourself a hole. I use the Chateau Cellar. Be sure and use the lower layers of your map.
Now on the ground layer of the map choose any entity you like as long as it makes sense.
I use the "plinth1" from model pack one. No it's not a default one but it does work well for this as a covering of the hole. It is just the right size for the single grid. Make sure it is positioned just right over the square hole by left clicking on the entity and making the adjustments. While you are there then make sure that three properties are set according to the image provided. Click on view. The main script used is the one I wrote. Just a two liner. Click on code below.
;Artificial Intelligence Script
;Header
desc = Death Trap
;Triggers
:state=1:destroy
:state=0,plrdistwithin=45:sound=audiobanksmashconcrete.wav,rundecal=0,state=1
;End of Script
Player distance does make a difference. Too close and the player can possibly get accross plank before it is destroyed. Too far and it will be destroyed before player even gets there.
You will notice you don't even have to put the strength to zero in the entity properties. That's becaue you are actually destroying the entity through the script and that ensures that the player will fall through. Take a look at the following views.
BTW, make sure and enlarge the image if your browser has amde it smaller or you won't be able to read the text.
As opposed to Highlander.