Originally Posted by Wikipedia

Whilst the motor vehicle is stationary and in neutral (N), the driver can select D for "drive" (after first pressing the foot brake pedal). The transmission's first gear is selected on the first shaft,[3] and the outer clutch engages at the start of the 'bite point'. At the same time, on the alternate gear shaft, the second gear is also selected[2][3] (pre-selected), but the clutch pack for second gear remains fully disengaged. When the driver releases the foot brake pedal, the outer clutch pack increases the clamping force, allowing the first gear to take up the drive through an increase of the 'bite point', and therefore transferring the torque from the engine through the transmission to the driveshafts and roadwheels — and the vehicle moves forward.