Torque Converter Operation At High Speed

As engine speed increases, the turbine speed approaches the speed of the impeller.
The fluid flow is directed from the turbine to the back side of the impeller blades.
The one-way clutch in the stator unlocks and the stator blades turn in the direction of engine rotation.
Fluid is no longer re-directed and torque multiplication no longer takes place.
This is referred to as "Coupling Speed". The turbine never reaches the same speed as the impeller as fluid flow would come to a halt. Ratio is approximately 1.1 to 1.

Fig 1: Identifying Torque Converter Operation At High Speed
Courtesy of BMW OF NORTH AMERICA, INC.