The ion pod was deployed in times when a starship was expected to encounter an ion storm, notoriously dangerous hazards to even the hardiest starships. Manually operated by one crewmember, the pod took readings otherwise not available to conventional sensors in the course of the storm. The pod's exposed position was a potential vulnerability to the ship (and the crewmember manning the pod) in severe ion storms that inflicted the ship with stress levels in the Force-7 range. In those instances, the ion pod could be jettisoned at the captain's discretion.
In 2267, standard ion storm/ion pod procedures of the USS Enterprise began with the order for the assigned duty officer to enter the pod as the distance from the storm closed. At contact with the storm's leading edge, the captain signaled yellow alert status. As the storm's chop increased pressure on the ship beyond natural vibrations (Force-2), and rose about the dangerous range of Force-7, the captain would order red alert throughout the ship. The new alert condition had particular import to the crewmember in the pod, giving him only seconds to leave the pod before the captain triggered the pod's jettison.
Lieutenant Commander Ben Finney used the opportunity of a severe ion storm and his turn on the pod duty rotation, to take revenge on Kirk for a broken career, on stardate 2945.7 in 2267. Finney faked his own death, and manipulated ship's computer records to show Captain Kirk jettisoning the pod before declaring a red alert. Kirk faced court martial on charges of culpable negligence for the incident, but was exonerated upon the discovery of Finney and his plot. (TOS: "Court Martial")