tom swift among the diamond makers

 the hull-down position was where the tank's turret was the only thing showing. kinda like when you're in an airsoft war and you have a sniper you want your gun and head over the wall this is the same thing the only thing that shows is the turret which is the most armoured part of the tank. and then it can also attack but not go on the offensive because as soon as he goes over that rimm his tanks underside will show which is the worst most less armoured part of a tank. so the other offensive tank will ya take no chances while blowing you up he will not hesitate to take that chance that's why I think if you are in the tank down position doesn't go and charge sit still and shoot at.