Rotor blades like to lop noggins in some types of helicopters and if you hit vertically with a straight spine you will risk some very serious damage as compared to being bent over.
This is what i would have thought would have been the best technique, however I read an article about an RAAF F-111 crew that ejected near New Zealand, the Pilot from memory had sat back in his seat with a straight back, the Nav wasn't prepared for the Ejection sequence, bent forward to check the Ejection handle and was in that posture when the Capsule fired. He Claimed the pilot didn't suffer any spinal problems, however he himself did.