In a scenario where you are about to be rear-ended, you should:

If you have enough time, tap your brake pedal. That will flash your brake lights to the driver behind you, and he/she will know to stop quickly. Worst case scenario, quickly see if the lane next to you is open (the shoulder is best) and use the free room to slow your car down in the open lane/shoulder. The goal is to get your car out of harm's way both from getting rear-ended and have enough room to safely slow down yourself, even if it's just a little bit of room, it will cause less damage to you and the car. That way the car behind you has more free space to slow down and you avoid rear-ending the person in front of you and getting rear-ended from the driver behind you.