Michael, you can achieve the same rotation of the torso with both hands in front in a normal fighting stance. There is IMO no additional benefit in pulling the hand back to carriage. In fact, as has been previously pointed out, by pulling the hand back to carriage for the reasons you are giving, that is additional rotation, you are leaving a huge gap in your defence. You will possibly get away with that in conventional sparring but you would very quickly be smacked in the head if you tried it in a close fighting situation.
I disagree. The issue is, the exaggerated movement is a training drill, but is not how you would actually fight. You would fight with shorter movements, the exaggerated movement is a tool used to develop a specific skill. In the Chinese arts, we often say, you start big to become small. The big movements help the skill become second-nature, but how it is used is a with a smaller (not exaggerated) movement. Because you used the big movement to train the skill, once you command that skill you can tap the same power using the smaller movements. But if you never train with the big (exaggerated) movements, it is much more difficult, if not impossible, to develop the full potential in that particular skill with only smaller movements. This is based on an approach that says, everything we do should be driven with full-body engagement. That's how we see it, anyway.
The rotation I teach, and this is what is taught at the Jundokan, is the hips begin the rotation. The torso moves next but with slight delay because the body is not locked. Next to move are the shoulders, again with that fraction of delay. (By this time the hips have actually completed their work and they are back centred.). The shoulders now provide acceleration to the arms which is the culmination of the power generation for the strike. Certainly while this is happening the other hand will withdraw but it will only return fully to carriage if I am pulling something in.
we would suggest you begin the rotation by driving the rear foot into the ground, which actually drives the hip thru the rotation. That brings the entire torso into the rotation. This, as you say, drives the shoulder forward and powers the punch. However, you are missing the other side of the rotation. Pulling the other hand back, with connection to the torso, helps power that rotation. When something pivots around a point, both sides of the pivot aid in that action. If you only push the punch out, you are only paying attention to one side of the rotation. When you pull back on the other hand (with proper connection to the torso), you are then working both sides of the pivot.
an analogy would be like a spigot that has two spokes that you grip to turn. If you only push on one spoke, yes you can turn the spigot but it's not so efficient. But if you grasp both spokes and push on one and pull on the other, the spigot turns more efficiently and with more authority. That is what is happening when you properly pull back the other hand when driving a punch forward. Again in the Chinese arts, we say, you go left to go right, you go forward to go back. This is an example of that concept. You go back to go forward. When you properly pull back the other hand, it drives the punching fist forward with more power.
The exaggerated movement you have described is also found in a couple of karate styles but certainly not Goju which is very square.
:asian:
Different systems manifest these ideas in different ways, and may use completely different approach. But in this discussion, people have referenced the Bubishi, which i I recall is a Chinese document? Well, i'm giving some perspective from one particular Chinese method, for what it's worth. I'm not trying to say, you need to do what I do or train in the method I train. But I'm giving perspective based on our approach to training, which simply makes a tremendous amount of sense to me.
again, for what it's worth.