To generate the fa jin, according to the traditional explanation, it is necessary to transfer qi from dantian towards the limb or body part (e.g. shoulder, head, hip) that will perform the technique with explosive force (bàofālì, 爆發力).
In terms of biomechanics, fa jin is a matter of utilizing body alignment and coordination to form an extremely efficient kinematic chain.
[2][3] In the practice of tai chi it is a description of a technique, generally indicating a sudden release of energy obtained by the coordinated movement of the entire body.
This technique is referred to in the forms of training school as the Lao Jia Chen Paochui (old frame, cannon fist, one of the two original forms Chen-style tai chi), while in Yang school, especially for those who have a direct lineage to Yang Shaohou the fa jin is added to techniques to release the stored energy.
When striking and squeezing the lower abdomen tight so that the core becomes compact, unifying the torso, this actively engages more of the muscle fibers to generate more force.