Thanks for that interesting input.
<<I've seen Ji performed well in forms and pushing hands much as in the photos above; also fingertips to wrist, palm to palm, and even without the other hand at all! I've been told that if the other hand isn't available, one still has to open both sides of the back when applying energy to the press, however.>> Polaris
I had not yet stretched my mind to encompass single-handed postures...
I wonder how applicable this concept would be for other stances in the form...
While a single handed An, for example, would probably work quite well, some other postures seem(to my unqualified eye) to require or encompass more fully the need for the dual efficiency of the co-operative yin-yang union.
In the case of Ji, as you explained, can I assume that the back is replacing the lost, missing arm in this interaction...as the supporting yin force?
Also, if reduced to single handed...Is it still considered Taijiquan technique?
It is definitely an interesting point you have addressed...Something I will definitely ponder and look for while involved in the mechanics of the form.
I've seen a palm-palm "Ji" myself somewhere before.
My personal instruction, experience has been the Yang style forearm Press as in the photo of Master Yang Zhen Duo above.
It is fascinating, however, to observe the different possibilities.