The standard piston ship consists of two parts seperated by line of FRME perpendicular to the direction of the required movement. On one side there is a piston in the direction of the movement which pushes the line of FRME and the other part of the ship away. To make things easier it only pushes the distance of one pixel. Then you have to delete the purple part of the piston that extended, this is usually done by CRAY but there are other ways. Now you end up with two seperated parts which need to come together again. To achieve that the other side of the piston ship we heven't talked about yet also has a piston which has to extend against the direction of the ship movement. As this second piston extends it pushed the line of FRME back so that it now touches the first part of the piston ship. To complete the movement The piston that just extended ahs to retract again and pull back the line of FRME and the first part of the ship. Then the ship has reached its initial state again and has moved one pixel in the required direction.