In a defensive engagement, the firing vehicle is positioned in a turret-down position. On command, the driver may bring the vehicle forward to clear the main weapon from the berm or battle position, moves into the hull-down position to engage target(s), then returns to the turret-down position.This is typically for Abrams, Bradley, and ASV crews, however when firing armedtrucks, the VC may wish to observe fires from the gunner and need to move forward into the “hull down” position. Basically the same thing rump is saying just added some pictures for examples. reference FM 3-20.21