In the back cast, although the rod hand will travel the same pathway as the front cast, it must travel, not only in the opposite direction, but up – against gravity.
The back cast is not simply the reverse of the front cast. One of the only similarities between my back cast and my front cast is that they are both initiated by a weight shift.
Due to gravity, the back cast requires much more power than the front cast. Instead of facilitating a 10-pound arm to fall, I now need to lift a 10-pound arm upwards for the back cast. In addition, a single joint at any one time to rotate the rod is insufficient to launch the fly line on an upwards trajectory.
In my back cast for accuracy, I use my shoulder, elbow, and wrist together to rotate the rod. Because I am small in stature and I am not particularly powerful, I need to use multiple joints to create enough power to overcome gravity and to punch my back cast upwards. I never want an anemic back cast. No one should.
The use of my wrist during the acceleration phase also helps to re-position the wrist for the front cast. It seems wasteful not to use my wrist during the acceleration of the back cast (when power is needed) so that, instead, it could be used at the end of the back cast to reposition the rod in preparation for the front cast.