Minecraft wiki entry for TNT_cannons, associated text: Using pistons to push the main charge closer to the shot also does the same thing as a condenser charge, more complicated to build but more controllable. To work, the main charge must be primed, turning the solid TNT blocks into entities. Once they are entities, a series of pistons can push and concentrated all charges into a single block space behind the shot. In this system, you get far more efficient power from a given size of charge.[5]