George S. Ledyard
Three, the ability to accept the energy of an attack and to join with it (ittai-ka or single body) physically is dependent on sliding the shoulder blades together without tensing the shoulders together.
I'm fascinated because it is the first time anybody associated with aikido has mentioned it where I could read/hear it.
Many years ago my myofascial therapy guy (who appears to have vanished into the Yucatan peninsula sometime before/during their last big hurricane) taught me how to isolate and use the muscles that pull the shoulder blades down and together from below, as a posture correcting maneuver.
I do find myself doing it on the mat as part of my preparation for training; now I'll have to think about why and what role it seems to be playing.
Do you have more information about it?