Master Chen Zhonghua Online Lesson on Apr 22, 2021 - Kelvin Ho
Fist of Hitting and Lifting
The flip from one side to another needs to happen because of torso going down. More precisely, the two shoulder-to-kua axes go up and down against the central non-moving axis.
There are two sides on the forearm as well.
In general, it is the idea of rubber that causes moving in another area. There has to be power conversion all the time.
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Source: https://practicalmethod.com/?p=75088