Re: Principles of pinning
for the ikkyo pin, as long as the palm is upturned (facing opposite the mat) it is gravity that's preventing uke from getting up. even with minimal contact (eg tegatana cutting ukes inside elbow, no 2 hands grabbing) uke cant get up. you need to prevent uke to turn his palm down.
in order to prevent uke from sliding away, the pin is also just slightly above 90 degrees with a slight bent in uke's elbow, the arm shouldnt totally be straight. one knee in the rib cage or shoulder pocket and the other in the arm.
If you get all the above details right I think you can use one or 2 fingers pushing in ukes inside elbow for control....I havnt tried it but I think it'll work given that I use only one tegatana, again if all the above criteria are met since you only help to make the conditions ripe for gravity to work its wonders. You as nage are only a small part in the pin.
For the other pins, it is more about uke damaging himself more if he tries to look for a release. case in point is the sankyo where nage makes a standing sankyo, uke tries to approach nage but damages himself more even if nage does nothing. This true for other pins as well.