The speeds and loads imposed on the fluid cause the impeller and turbine blades to shear, or slip, through the fluid to a certain degree. This fluid slippage causes some inefficiency, especially at higher vehicle speeds. The engine can run faster than the turbine or output shaft, thus wasting fuel. To eliminate this inefficiency, many torque converters provide a direct mechanical link (called lock-up) between the engine and transmission. At lock-up, the turbine and impeller turn at exactly the same speed. There is no fluid slippage, which helps to reduce heat build-up.

A lock-up converter is one of the most common ways of providing this mechanical link. A lock-up converter mechanically links the turbine to the converter cover at various operating speeds, depending on vehicle model and driving conditions. The cover is mechanically bolted to the engine. At lock-up, the converter cover drives the turbine. The hydraulic link is eliminated, and the engine and turbine are mechanically locked together, directly driving the transmission input shaft. A lock-up converter requires a clutch to engage and disengage the mechanical link between the engine and the torque converter cover.

Two major types of converter clutches are the centrifugal clutch and the hydraulically applied torque converter clutch.