Defense in sector maximizes the combat abilities of the infantry. It allows the platoon to fight throughout the depth of the sector using dispersed small-unit tactics.
Defensive Warfare Strategy might have a Security Element (consisting of an Assault Fire Team patrolling the flank), Assault Element (consisting of a Heavy Weapons Fire Team providing the 'shock and awe' which is denying enemy movement), and Support Element (Safe House, where wounded troops are getting patched up and are on their way back to resupply).
From the US Army Small Unit Tactics Field Manual
(1) The platoon is usually assigned a sector within the company sector. The platoon leader may in turn assign sectors to individual squads to permit maximum freedom of action for the squad to defend. The platoon leader must remember that the squad has no way to call for fire support other than through the platoon net. FOs may be attached, or as a minimum leaders must be prepared to assist in calls for supporting fires.
(2) Each squad conducts detailed reconnaissance of its sector and identifies all likely enemy avenues of approach, choke points, kill zones, obstacles, patrol bases, and cache sites. They also identify all tentative positions.
(3) The platoon leader confirms the selected tentative sites and incorporates them into his concept (Figure 1-4). He designates initial positions and the sequence in which successive positions are to be occupied. He gives each squad specific guidance concerning contingency plans, rally points, and other coordinating instructions.
(4) Squads then prepare the defense in the sequence designated by the platoon leader. They initially prepare the primary position and then a hasty supplementary position, and then they select the alternate position. Squads improve the positions as time permits.
(5) When Security warns of approaching enemy, the squad occupies its primary positions and prepares to engage the enemy. As the enemy moves into the choke point or kill zone, the squad initiates an ambush. It engages the enemy targets only as long as squads do not become decisively engaged. Squads then move to their next position and repeat the same process.
In LSN, the Security Elements may be as small as a single Grunt in a room cornerdancing, or two Grunts within a house along the flank. The concept is to have a warning when enemy forces are moving, and to be able to react with a quick and bludgeoning counterstrike.
(6) Casualty evacuation and resupply of ammunition are particularly difficult when defending this way. Having a strong Support Element and rotating troops back to aid stations and ammo dumps is critical to success unless you are able to wipe out the enemy quickly. Keep your forces within ten seconds of the safe house, where they can get patched up.