Minecraft wiki entry for Piston, associated text: When a slime block is pushed or pulled by a piston, while moving, adjacent blocks also move with the slime block, unless a non-piston movable block stops the blocks that are "grabbed" by the slime blocks. These blocks may in turn push other blocks, not just the blocks in the line in front of the piston. For example, a slime block sitting on the ground attempts to move the ground block underneath itself, which in turn has to push additional ground blocks in the direction of motion just as if it were being pushed directly by a piston.