An explosive override was a manual control that triggered explosive bolts which forced the opening of cargo bay doors. The override, located in the bay, was accessible after a panel had been removed. The override was used when automatic controls on the doors failed to work. Klingon Birds-of-Prey were equipped with this feature.
In 2286, as the HMS Bounty was sinking into the San Francisco Bay and the humpback whales George and Gracie faced the prospect of drowning, James T. Kirk, risking his own life, used this override to free them and himself from the ship. (Star Trek IV: The Voyage Home)