Minecraft wiki entry for Logic_gates, associated text: Extension: for those new to advanced redstone like myself, it's easier to understand it like this: let's say output B (C) has a NOT gate that inverts the signal and it leads to an iron door or piston door etc. Output A (S) is connected to sticky pistons controlling the floor. Let's say for sake of argument that there is 1×1×1 block NOT affected by the sticky pistons, this is the safety block. When you activate input A, both the door will open and the floor will drop, if you're standing on the safety block, then you will not fall. Input B will control only the floor, but if input A is on the input B will control them both. When both are on, input A will only affect the floor. This means if you are off the server and want no one in, leave A and B on, when they deactivate A, the floor will drop, but the door will stay closed, so if they know the secret, they still cannot get in.