A lot of students learn sticking hands as a fixed movement and response. This does not work against a non-wing chun person who moves and responds differently.
In addition to sensitivity, on off, penetration etc. covered in the other chi sau clips the body and hands have to work independent but supportive of each other in order to achieve a high level of chi sau. Each does its job but the jobs are different like a pilot and a flight attendant both contributing to a safe flight or handling an emergency in flight. This is hard to do and even hard to perceive when it happens right before your eyes.
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