Minecraft wiki entry for Traps, associated text: The "C4 trap" is a vertical or horizontal stack of TNT with a button wired to a dispenser with a fire charge in it. The dispenser is placed one block above or below the stack (vertical) or one block next to the stack (horizontal). This usually works better for demolition than for a trap. If you want to use it as a trap, wire a pressure plate to the dispenser. This works similarly to a flaming arrow hitting a stack of TNT from above. This works better than a dispenser shooting TNT, because the "C4" stays in one place. Also, if only two TNT blocks are used, it works well as a disposable cannon, even better on obsidian blocks, but not in a tube. All evidence of the trap is usually destroyed. Note: C4 trap from below does not work well.