Final answer:
The HQ element ensures command and control, while the trail fire team provides rear security after a halt has been signaled. These actions are part of the Immediate Action Drills that allow military units to respond to situations without direct orders.
Step-by-step explanation:
The question refers to the procedures followed by a Headquarters (HQ) element and a trail fire team after a halt has been signaled in a military context. When a unit halts, the HQ element typically takes on a role of maintaining command and control, ensuring the overall security posture and readiness of the unit. Meanwhile, the trail fire team, which is the last element in the movement formation, is responsible for rear security. They would immediately assume a defensive posture and cover the unit from any potential threat that might approach from the back. The HQ element will effectively communicate with all parts of the unit to confirm their status and direct the establishment of security measures such as local security teams or observation posts, as well as coordinate the next steps for the mission.
These actions are part of the larger Immediate Action Drills that military units practice to respond quickly and effectively to different tactical situations without the need for direct orders. It ensures the safety of the unit during unexpected halts due to enemy contact, obstacles, or other interruptions during movement.