The Hull Down position is the position of hiding the body of a vehicle but leaving the upper part uncovered. By doing so, tanks gained cover and became very difficult to defeat while still maintaining their offensive capability despite being static.
This was usually achieved by deliberately digging trenches into which the tank could sink. They were also followed by a walkway deeper than the surrounding ground level so that they had cover not only when approaching the trench but also when retreating from it.
Tanks with greater mobility in the turret were quite capable of controlling a large expanse of terrain while still under cover. Normally in the turret and near the gun most of the armor was placed. In the Hull Down position the enemy was forced to fire at the most resistant areas of the tanks.
Normally this military tactic was used to defend against attacks because the positions of the tanks had to be premeditated and respecting the cover blocked their movement. The most typical way to use this tactic was to lean the tank over an elevated hill and only take out the cannon. It was also used by entering over a depressed area or placing the tank behind a terrain element such as a building that allowed a glimpse of the cannon.