The breathing technique [sink the chest, raise the back] described below by Yang Chengfu has always been a bit of mystery to me before and denied by many to actually exist, perhaps in reality few ever learnt it properly. Li Yiyu previously attempted to explain the method in more detail and how it is actually applied in combat.
含胸拔背:
含胸者,胸略內涵,使氣沈于丹田也。胸忌挺出,挺出則氣擁胸際,上重下輕,腳跟易於浮起。
拔背者,氣貼於背也,能含胸則自能拔背, 能拔背則能力由脊發,所向無敵也。
楊澄甫口授 陳微明筆錄
Sink the chest, raise the back:
When sinking the chest, the chest will form a shallow hollow, this helps focus the Qi into the Dantian. One mustn't raise the chest outwards otherwise the Qi can [unexpectedly] rise into the chest and cause one to be top heavy and the root of the foot to become too light.
When raising the back, the Qi will "stick" to the back, and [of course] its only possible to raise the back if the chest is sunk; if you can raise the back then its possible [to learn how] to release power from the spine, thus making one a formidable opponent.
Yang Chengfu's oral transmission recorded by Chen Weiming
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力從人借,氣由脊發。胡能氣由脊發? 氣向下沈,由兩肩收於脊骨,注與腰間。此氣之由上而下也,味之合。由腰行於脊骨,布於兩膊,施於手指。此氣之由下而上也,味之開。合便是收, 開即是放。能懂得開合,便知陰陽。
李亦畬
Force is borrowed from the opponent, Qi is discharged from the spine. How can Qi be discharged from the spine? The Qi is sunk downward, then from the two shoulders, is drawn into the spine, being concentrated into the waist. The Qi moving from above to below, is known as Closing. From the waist the Qi is drawn into the spine, passes through two arms and out through the fingers. The Qi moving from below to above, is known as Opening. Closing is to gather, and Opening is to release. Understanding Opening and Closing, means knowing Yin and Yang.
Li Yiyu