This is the result of a specific design project. Unlike the commoner automatic transmission systems, the torque converter is calibrated to a particularly high stall rate, due to the fluid dynamic coupling characteristics.
This means that the design allows relative slip between the engine and the transmission up to a higher rev rate than is ususally acceptable.
The result is a very pronounced multiplication of the torque, which translates, for the driver, into unequalled acceleration and elasticity.
To improve the overall effect, above certain speeds a mechanical clutch intervenes to exclude the hydraulic coupling in long ratios. This eliminates the loss of power normally dissipated by converters, consequently reducing consumption without penalising performance which is practically the same as on the version with a mechanical gearbox.