Exactly!From my experience this is the most important part of an imobilization.Make sure that the shoulder is pinned on the mat (on the ground in the case of a real fight) and that it stays there!
I also like very much simple approach, following principle KISS. However this will be too simplistic, just pin the shoulder. As in aikido there are no rules, attacker can do literally anything to go out of pin. That why I appreciate Michael and Kevin input, because their real life experiences follows similar 'no rules' scenarios. In this context, I like to think that from technical point of view the control of whole body structure is a minimum as opposed to concentrate on only one point of the body.
Real life experience tend to demonstrate that such control by one person can be achieved only with big difficulty and carries out a lot of risk. Nonetheless in aikido it is some kind of standard and it is approached as an easy routine.
But this discussion can give us some partial answer for question HOW.
Now, still we should ask question WHY.