find a route forward through the relative safety of hidden low ground (dead ground). If crossing a long crest is unavoidable, the vehicle can back down and jockey at least 50 metres across the covered back of the slope, before advancing over the crest at high speed. An enemy gunner will have little time to locate the target, train his sights on it, and take the shot. If the terrain is hilly enough, the AFV can quickly enter low ground, then advance through it to another hull-down position. Mutual support Small armoured units (companies or platoons) make use of these tactics in co-ordinated fashion, when contact with the enemy is expected. Since firing while moving was until recently impossible or ineffective, elements of a unit (platoons, patrols, or individual vehicles) take turns moving and supporting each other from the halt (see overwatch). This is called mutual support, or fire and movement, related to the infantry tactic of leap-frogging, or, somewhat more loosely, the air combat tactic of flying with a wingman. Co-ordination is accomplished by hand signals or radio messages. Lightly armed reconnaissance elements make much use of covered movement and stealth, while offensive units such as tanks move much more aggressively. When speed is paramount, modern tanks (which can fire effectively while moving) may dispense with fire and movement, and move all at once. Tank design Tanks and other fighting vehicles must be able to depress their gun to be able to take advantage of many hull-down positions, since a vehicle's hull is usually tilted upwards when it is behind a crest. A vehicle with a relatively small range of gun depression may have to drive up onto an exposed crest or forward slope to be able to fire on lower ground to the front. Notably, Soviet and Russian tanks after World War II have very low profiles, but pay for this advantage by having a poor range of gun depression. Their low turret roof stops the rising gun breech when the muzzle is depressed. Thus, Soviet tank crews would have a hard time finding a hull-down position from which they could cover much of the terrain by fire. The typical Soviet tank had a range of elevation of -5 to +15 degrees, about two thirds that of Western tanks with a range of about -10 to +20 degrees. This disadvantage was deemed acceptable, as Soviet tanks were designed