Tanmeiryu, I think that my understanding coincides with what Bob has said. Put another way, the intent is to attack a weak spot using the maximum internal structure.
<font face="Verdana, Arial" size="2"> in my other studies i learned that a back fist was performed with the knuckles leading the strike. this is done so that the knuckles strike first and to protect the fragile bones in the hand.</font>
I think this is excellent logic; however, I also think this is different logic from our Tai Chi approach. We try to “use soft to overcome hard.” We also try to use “soft and hard combined.” Hard should come from soft, and the soft should come from maximal integration of responsive components. If you change only the wrist angle, that kind of integration is no longer possible.
If you look at the strikes in the form, you may notice that none of them really requires attacking a hard target with a hard surface. You may also notice that the hand structures are chosen with a view to an integrated natural structure. Even the fists are not meant to be held in a completely rigid way, since that would violate the principle of getting hard from soft.
In some martial arts and even some versions of Tai Chi, there is a deliberate attempt to reduce the striking surface of the fist to get more “penetration.” In our Tai Chi, I perceive such an attempt only in Double Peaks to the Ears, which is a one-knuckle attack to the temple. Even there, however, the strike begins from the hip, travels in a large circle, and is closely supported by the legs, the weight shift, the angle of the torso, and the arm shape. The attacking knuckle is also simply part of a natural fist, rather than an artificial extrusion supported only by muscle power.
Another consideration is that the form seems to stress training internal structure over external structure. In Chop with Fist, we strike in line with our shoulder and foot, rather than in line with our nose, as might be expected if you imagine standing squarely in front of the opponent. Both arms come from a position of forming complementary curves outlining a large vertical circle in the left front of the body and then switch to corresponding curves, forward and back, at slightly different heights. Notice that the right arm goes from having a low curve with the inside of the wrist on the outside of the curve to having a nose-level curve with the outside of the wrist on the outside of the curve. Some of the “internal power” comes from the complete inversion of the curve and would be lost if the wrist did something on its own to change the angle of the knuckles.
A last observation might be the relationship between Chop with Fist and White Snake Spits out Tongue. We do the latter movement as a finger flick to the eyes. Such an extension of the back fist would not be possible if the wrist were seated with the knuckles leading the strike. Also, the logic behind our version of the strike is that the opponent has tried to tilt his head back away from the backfist. We then adjust by straightening the right wrist further and unfurling the fingers to strike the eyes.
I hope this helps.