Definitely a multiple exposure of some type. Either masking her face and doing it in-camera, or later in the darkroom. I vote for the latter, given the symmetry of the duplicated areas in the hair.
This has bugged me for many years and it's probably a dumb question with an obvious answer....
Has any one figured out how he made that Bridget Bardot portrait with sharp features and hair thats sharp but moved somehow?
Love to know........
OK, he kicked the enlarger! However, was quite serious about Steichen kicking out. I can't remember which book I saw it in but there was a colour studio shot which had a similar shuddering look. Tried it myself with 5x4 and has to be done quite gently! I know the Bardot shot and right about the eyes. Perhaps a slight double flash with the hair swishing about from a fan or assistant nearby wafting a big board?
Shake n Bake: Makes me think of what was being made by commercial shooters I know back in the 90s. Long exposures, one or two flash pops, smack the camera in between the two pops or after the first to have blurry edges.
It didn't seem that Avedon was terribly interested in darkroom techniques, so I also think it was in taking stage. Could it be multiple exposures with zoom lens, or just moving back? I was surprised that the "Assisting Avedon" assistant didn't include any tips/tricks on this portrait. Some of the other portraits that he did later also includes multiples of hair yet face in focus.
I've seen some of his working proofs for prints - he was extremely involved in his printing, certainly in the later years of The American West work. He may not have been a darkroom technician himself, but he was very concerned with finished prints and would mark sometimes very small areas for +1/3 stop burns or -1/4 stop dodges.