Tai-Chi essential 9 of 10: 相连不断 (Continuous movements)
相连不断 外家拳术，其劲乃后天之拙劲，故有起有止，有线有断，旧力巳 尽，新力未生，此时最易为人所乘。太极拳用意不用力，自始至终，绵绵 不断，周而复始，循环无穷。原论所谓“如长江大河，滔滔不绝”，又日 “运劲如抽丝”，皆言其贯串一气也。
My rendition of essential 9: “During practice, all movements must be joined together seamlessly, from beginning to end, with chi smoothly increases and smoothly decreases in repeated cycles without any breakage or interruption.” This is also called "practice chi-muscle movements (运劲) like drawing silk”.
Paul’s additional comment: in actual practice, there will be a trade-off effect between the structural essentials (essential 1 to 5) and the flow essentials (essentials 6 to 9). The trade-off is in particular marked between 1 to 5 on the one hand and 9 on the other hand. What I mean is, oftentimes, a pursue of the structural essential may lead to a relaxation of the flow essential (of course, in a perfect form practice, both need to be taken care of). It is advisable for a practitioner to separately focus on either one during one’s elementary and intermediate levels of practice.