The emergency overload bypass, or emergency bypass control, was an engineering component aboard Constitution-class starships which acted to prevent a catastrophic explosion in the event of loss of magnetic containment. It was accessed through the matter-antimatter integrator control, located just off the main engineering section.
In 2268, Losira managed to sabotage the Enterprise's engines by fusing the bypass control, causing the ship to accelerate to wild and uncontrolled speeds. The defect was only corrected with Chief Engineer Scott managed to manually disrupt the antimatter flow. (TOS: "That Which Survives")