SI-Drive 2008+ STi Explained:

Manual Mode

The six manual differential locking settings remain the same as in the previous model. In manual mode, DCCD allows the driver to vary the front-to-rear torque distribution (up to 50:50 maximum) to optimize AWD performance for varying driving conditions. The driver first selects manual mode with a console-mounted button and then pushes or pulls the rocker switch on the center console to select from six levels of center differential locking.

Increasing the lock factor sends more power to the front wheels, which the driver might want in certain driving conditions or to suit an individual driving style. A helical-type limited-slip front differential and TORSEN* torque-sensing limited-slip rear differential optimize power distribution side-to-side, helping to prevent the inside wheels from slipping during cornering.