HoRo1, you simplified a point I think you guys are missing from the EMT's quote. You are correct, you don't have time to duck. I rolled about as slowly as physically possible in my friend's truck and I didn't even have time to cuss let alone react and move my body to a safer position. Even if I was fast enough to voluntarily duck, nobody in the world is strong enough to overcome the forces of even a very slow wreck. Try bracing yourself in your buddy's car and have him hit the brakes as hard as possible to get and idea of what a wreck may remotely feel like.
What the EMT means by "ducking" is your body is forced by the roof (or ground in a convertible) out of the 3pt belt. In a static shoulder harness setup, your body can't move and what ends up giving is your neck... which snaps. Think about reaching for the glovebox in a 3pt vs shoulder harnesses. You have enough "give" left and right with a factory 3pt to get squished up in the car without turning your neck into an "L".
BTW, in my very slow roll (toyota pickup) I ended up with my head pressed against the ceiling and my body crooked in the seat. Would NOT have wanted a shoulder harness!