Re: The shoulder grab - where does this come from
Q2. yes - I grab whatevers there. If I'm Uke I'm going to try to jam the shoulder to prevent Tori from punching me with that arm. As others point out ..in the dojo it'll be accompanied by menuchi or something similar...outside its more likely to be a punch to the face, elbow or glasgow kiss if you are in joe's neck of the woods.
Q3. where does it come from? Who can say....most of these attacks would come quite naturally in any attack on the world from people without a jot of training.
Personally I'm more interested in HOW the attack is dealt with than a historical source, but thats just me.
I've seen it dealt with in many ways....different yet ultimately similar in that uke beats the arm and then uke up. Ive seen short stick defence, yawara bo defence, karate defence....all hitting the arm or pinning the hand and then moving on with a number of strikes or a lock...or a a takedown.
These are all interesting, effective methods of dealing with this....but as an aikido person I'm interested in using Aiki...we all are right? so wheres the Aiki way of dealing with it that differs from jujitsu, from karate etc?
Others have mentioned push and pull...forces. A lot of my historic practice has been around moving away from a push or into a pull.
I look at moving away, or turning as tenkan....if I combine this with a rotation around my spine and irimi/atemi on the other non held side...I see yin & yang.........some could call this combination a union of opposing forces in me.....and then for me as an Aikido person it starts to get interesting.....using more than just beating up uke to find solutions.