The Mythbusters TV show in the US did a real life experiment, into the use of the crash position. Now, they were using a mock up aircraft that was suspened about 13 feet in the air, and impacted the ground at about a 60 degree angle. They used a dummy with G meters and other gadget's.
They actually found that the brace position reduced injurys upon impact. And increased survivability of the occupent. You would really need to watch the episode to get all the info they found. And believe me, these two guys on this show are the least scientific two you will find on the planet. But, they doo get some good insight into the Myth of the Brace position, and it's pros and cons.