重心轉移 + 用腰
is much like
"自身是以分清虛實和重心偏移不來産生偏心力矩作爲力量源泉。也就是說,太極拳的虛實一定要掌握分寸,虛實變化産生重心移動這是産生動力的基礎。上面所述的前弓後蹬發勁要快,始終注意自身重心下降和平衡,這樣就是靠兩腳的移動,隨時地變換虛實,並協調身、手等各部位,以保證身體平衡和偏心力矩打擊對方。"
Of course, this is helping me to understand it using my past experiance, I cannot do it well myself!