Double domino's are relatively hard to explain so you should watch the video to get a good idea of what it is.
This is possible because the bricks are very wide. when the bricks fall, they lay on top of the one before it. the last brick in the sequence does not have anything to lay on so it falls to the floor. this causes the brick that is laying on it to fall as well and the next brick to fall and so on. This happens only when the bricks are placed a certain distance away from each other. this distance cant be too close or the bricks will just rest on top of each other. this distance also cant be too far away or the bricks will lay flat on the floor after hitting.