I agree that teh description sounds confusing. That's why I referred to the books, but they may be hard to find.
Imagine the normal shiho entry from yokomenuchi. In Iwama it is said that the grasping hand takes hold of uke's wrist with the nage's thumb on the pulse. As a result of this, uke's arm is twisted inward. Nage's free hand is formed as tegatana and braces the crook of uke's elbow. Nage then makes an ura/tenkan movement, similar to Ikkyo, while holding uke's arm in the same position. When uke lands face down, nage positions his inside knee at uke's armpit. Nage pins uke's elbow to the ground, at an angle rearward of perpendicular and draws back the outside knee at the same time as drawing uke's wrist/hand rearward. This creates tension on the shoulder joint.
I hope that clarifies the mechanics of it. However, I think what is important is that rokkyo and nanakyo are now classified in Iwama as Osae waza, pinning techniques. These go right alongside the traditional pinning techniques of Ikkyo-Gokyo. The focus in all of these techniques are the final pin, not just the entry and take down.