The system only reacts for vehicles approaching from the sides when you shift into reverse (R).
If the system detects a vehicle approaching your vehicle, it provides a warning through the cross traffic alert system. See
What Is Cross Traffic Alert.
The system only applies the brakes for a short period of time when an event occurs. Take action as soon as you notice the system applying the brakes in order to remain in control of your vehicle, the system does not do this for you.
Note:
You need to enable cross traffic alert for the system to function.