this is very interesting to me...though I don't know much about the biology involved
I recently started studying tai chi and one effect that it's had on my aikido is that my shoulders have become much more relaxed during aikido waza, consequentially there's less opportunities for resistance by uke
Good observation! We engender resistance in the uke as a direct result of our failures. When we use certain muscle groups and/or utilize muscle contraction to freeze shoulder movement, we have essentially allowed any force going through the shoulders to directly effect our spine. Any time we respond to tension with reciprocal tension, we also create a reactive response from the uke.
If we can allow incoming force to go cleanly through our shoulders and dispersed in multiple directions, the force does not effect our posture, balance and movement. The incoming force will also continue until the expected feedback (reciprocal tension) is received, signaling the necessity to change actions. This is essentially what you are describing in the lessened opportunities for resistance by the uke.