This circuit basicaly does an explosion WITHOUT an explosion - it destoys just one wall and it does not use gunpowder keg, match, or detonator. It was discovered by mgblitz81 (level-pro SC's electrician). If you wonder how is it possible, download the world file, or look at official topic.
When you will press the button (you cannot see it on the picture, but it's connected with the wire leading to the bottom side), trapdoors will open and (after 0.25 sec) close. The gravel wall will be destroyed. NOTE - there are just two blocks of gravel , but normaly there should be how many you want.
Usable in -
This circuit comes very handy when you need the wall to disappear, but you don't want to use gunpowder kegs (because they will destroy whole room).
As of V2.0, this is done easier with pistons.