Milipedia wrote:

Defilade is also used to refer to a position on the reverse slope of a hill or within a depression in level or rolling terrain. Defiladed positions on hilltops are advantageous because they allow a defender to take advantage of the height of the terrain without suffering the disadvantage of being silhouetted against the sky. However, because of the slope, "dead space" that cannot be engaged with direct fire will be created in front of the position. Ideally this dead space should be covered by the interlocking fields of fire of other nearby positions, and/or by pre-planned indirect fire such as mortars or other forms of artillery.

In the case of antitank weapons, and especially short-range man-portable antitank rockets, defiladed positions behind a hill have several important advantages. This is because the dead space created by the intervening crest of the hill prevents an approaching tank from using the range of its direct-fire weapons, and neither the attacker nor defender will have a clear shot until the tank is within range of the defending antitank weapon. In such engagements the tank is usually at a further disadvantage because the defender will often be camouflaged while the attacking tank will be silhouetted against the sky, giving the defender an easier shot.

In addition, if the tank fails to detect the defending antitank weapon while the tank is still defiladed, but advances beyond that position to the crest of the hill, it may expose the relatively thinner armor of its lower hull or belly to the defender. Early detection and elimination of antitank threats is an important reason that tanks attack with infantry support.