When someone is about to hit your car from behind

I think it would be better not to brake, but to remove the legs from the pedals - you may break your right leg in the accident.

You might break your leg but if you'll be driven into a tumultuous junction you might get killed by another side collision -try lifting the hand brake. Another tip I can give you and it is a general one as well is hold the steering wheel with your thumbs pointing to the out side. If you hold the steering wheel with your thumbs closed and pointing to the center of the wheel you might break them when hit from behind . When the thumbs are on the outside your entire hands will slide forward .

I agree with the others.  Unless being propelled forward would put you in additional dangers, I wouldn't brake.  Instead, sit upright and lean back until your head pushes on the head restraint (head rest).  That way, when the car hits you from behind, your head won't slam back and you won't have whiplash.  And keep your hands on the outside of the steering wheel.  If you are bounced into something and your airbag deploys, it will break your hands/fingers/wrist if they are in the way.

First and foremost, you never stop behind another vehicle without giving yourself enough room so you can turn hard right or left to get away from a potential rear end collision. Even if you don't get clear in time, you will be at an angle to an impact, which will reduce the force of impact on your car.

Next, when you are stopping behind another car, with none behind you, you must check your suruondings for where you could go, if needed.