Re: Principles of pinning
To address the slight digression to the use of pins in protective services, I'll say that you need to change your approach from traditional methods. I used to do demos with cops (good judo players, MMA people, experienced officers) and to prove a point I would tangle with them and while they wrestled for an arm bar or hip throw I would secure their side arm, baton, or another item from their practice belt. When I work with people who carry weapons I usually advocate not to get entangled with their opponent. So in this sense I think persons who work in protective services like police, security, national guard, military, etc. need to understand their first priority is to maintain their security, then secure their partner. As far as I am concerned, There is not an excuse that should be considered that allows a suspect to touch a police officer after the officer begins issuing submission commands, certainly one who is armed. Cops have such dangerous jobs its not even funny anymore...
Back to the larger discussion...
Pins are controls; at the end of the day, I ask myself, "am I in control of this person?" This is my defining characteristic of an aikido pin. Seriously, I've had great aikido people grab my hand and stop me from tenkan. Not a traditional pin, but if I cannot move how I want to...
Pins are not necessarily breaks, joint locks, or pressure points, although all of these options are pinning tools. Traditionally, a pin was a pause in combat that shifted a significant advantage to a combatant to apply a killing or debilitating blow to his opponent. This would be a similar tactic to kicking sand, standing with the sun at your back, or any number of combat tactics that are designed to give advantage. Interestingly, several of my historical books tend to imply that it was unusual for combatants to engage in single combat on a battlefield; rather, the focus of single combat was incredibly dangerous on a battlefield with multiple opponents. Japan was sort of odd in this respect and the challenge of single combat gave the Japanese trouble in several foreign wars. As it applies to combat and the arts that study combat, I believe Aikido differs from traditional [koryu] pins because of my intention in pinning my partner.
Sport pins are designed to demonstrate that advantageous pause. Some sports choose body control, others a duration of time of control, others a submission by the opponent. But even the sport pins are designed to show that one player has advantage over another. I believe aikido differs from sport fighting pins because the rules of engagement are different.
Aikido walks the line between these worlds. We tend to adhere to concepts of combat and organized military strategy, but often our training is sport-oriented because we do not act as if we were on the battlefield (must fight ikkyo...without...regard...to atemi...). But that is a problem with our training, not necessarily our pinning.
Now, in response to your comment about the use of force in submitting suspects (in your protesters example). I think we need to respect the social pressure for law enforcement to go to [in some cases] extreme solutions to minimize the risk of injury in subduing persons. Like I said earlier, my tolerance ends after the officer commands the suspect to submit. If the officer never had to touch the suspect, there would be no risk in the officer injuring the suspect... When I bounced the way we took down big brawlers was to dog pile them and smother them to reduce the risk of injury to the person, the patrons around him, or damage to the establishment.
Pins have a consequence for non-compliance, that consequence is often discomfort at best and can be unconsciousness, skeletal injury, muscular injury or nervous system injury. These consequences are often too severe for civilian enforcement groups to entertain so traditional pinning solutions may not work for these groups.