Tai-Chi essential 2 of 10: 含胸拔背 (Back with chi)
含胸拔背: 含胸者，胸略内涵，使气沉于丹田也。胸忌挺出，挺出则气拥胸 际，上重下轻，脚跟易于浮起。拔背者，气贴于背也，能含胸则自能拔背， 能拔背则能力由脊发，所向无敌也。
My rendition of essential 2: “During practice the sternum must be in a state of relaxation, and a practitioner must drive the chi up his (or her) back” After defining the centre-line in essential one, essential two deals with strengthening this centre line using chi from lower abdomen (dantian).
Paul’s additional comment: for best results, this essential act must be performed together with another important act called 气沉丹田 （"Generate chi from dantian 丹田"） And the way to generate chi from dantian the tai-chi way, reverse abdominal breathing should be used (check breathing method here). Both essential one and two should be performed together during practice – defining a core-unit of practice technique. This essential is similar to Core strengthening in Pilates.