Minecraft wiki entry for Basic_CSGs, associated text: Design A is most basic of all (from this video) and safer, although it can lose some cobblestone to the lava unless a slab is placed over the lava. Design B (from this video) washes the mined cobblestone toward the player, so as to minimize loss. When building design B, you can leave a block where the cobblestone will form, and mine it to start the process.