Re: Shihonage roll-out
We were just working on this pin last night, and found that, in addition to much of what has been mentioned above, that the angle that nage forms to uke's body makes a big difference. This is because it affects the vector of force that nage's forearm makes to uke's upper arm. If nage is, for instance, 90° to uke, the forearm is pushing down, but also more or less away; any slight attempt at a roll will create slack, allowing further roll and escape. But if nage is 45° or so to uke, the vector pushes down and towards the top of their head. This rotates the elbow towards the mat, and not so much across uke's head. And all this, of course, is in addition to pinning the shoulder, taking the hand in a spiral down and away from the head, lining up your weight on the pinning hand, etc.
We were working mostly on a standing pin, but the same variables would apply to kneeling, with or without using uke as a knee cushion.