I consider that a false difference, KFW. Hands can remain at guard, pull back for next strike, or be static, regardless of the system.
When you punch right hand, you can use your left hand to
- remain guard to protect your head,
- block your opponent's punch,
- guide your opponent's arm away,
- pull your opponent's body toward you,
- ...
When you are "not doing any of the above", you should
- pull your left hand back,
- cause your body "rotation",
- extend your body to the maximum, and
- generate your maximum amount of power into your punch.
IMO, the mind set are different. When a
- boxer thinks about right punch, he thinks about to send right arm out. The body rotation start from right arm out.
- CMA (or Karate) guy thinks about right punch, he thinks about to pull left hand back while send right arm out. The body rotation start from left arm back.
This can be seen in the following clip. You can clear see the sequence of his
- left hand back,
- left shoulder back,
- right shoulder out,
- right hand out.