That's what I get for posting past 1am! Nobody is right or wrong here...at least for me anyway. It's just that Graham felt like sharing so I said, "why the hell not???"
I focus on the pinky and index finger as well...sorry for the confusion. What I was getting at is, it's also that they are aligned with the bones. Then as a result, the connection is pure.
To think of the 2 forearm bones as entirely as one unified arm is good in it's own way. But you can also think of them as individuals or independent. Yang and yin ...or full and not full...whatever gets you there.
Think of 2 ropes laying side by side...even with some tension pulling them taught, they are still independent of eachother. Now twist the bones(ala aligned with and using the pinky or index fingers) and affect the change you desire in a posture, an application or even in qi gong. The bones twist and connect. It's like open and close...with the forearm bones (with peng/expand and relax)
Even the "tigers Mouth" has open and close in my form. For us in Sun style, you'll see the "tigers mouth" is a bit more closed in Bagua...with the center of the palm being more hollow and deep. In Xing Yi, it's far more open and flat. In Taiji it's kinda middle between those 2 more relaxed. Much more for me personally, since I concentrate moreso on my Taiji...but my Tigers mouth definitely opens and closes throughout the form.
For us when the connection is pure, (or potent, or alive...) all the way to the finger tips...then it will be also correct down to the feet = upon testing.