//This is inside update
if (Input.GetKey(KeyCode.Z)) {
    canReinforcements = true;
}

if (canReinforcements)
    StartCoroutine (callReinforcements ());


//Then coroutine

public IEnumerator callReinforcements() {
if (reinforcements.transform.position.z < -3.74 && canReinforcements) { //if below screen
    reinforcementsBoxCollider.enabled = true; //enabled box collider since it's resting position is on lower boundary
    reinforcements.transform.Translate (0, 0, -2.5f * Time.deltaTime); //move up from below screen
}
yield return new WaitForSeconds (10f);
canReinforcements = false;
if (reinforcements.transform.position.z > -10) //if above screen
    reinforcements.transform.Translate (0, 0, 2.5f * Time.deltaTime); //move below screen

reinforcementsBoxCollider.enabled = false;
}