HAC - Hill Start Assist

This is Toyota's start-off assistance device, which is one of the traction enhancement systems.

The device causes the brake control computer to automatically activate the 4-wheel brakes for a few seconds if the driver releases the brake pedal to prevent the vehicle from moving backwards, thus facilitating the same restart on an incline. In fact, as soon as the driver releases the brake pedal to engage the accelerator pedal, the HAC control system automatically applies the brakes on all four wheels for a maximum of 4 seconds, thereby preventing the car from rolling backwards and thus providing more traction. ...

2010 4Runner How-To: Hill Start Assist Control (HAC) | Toyota