This refers to the sticking aspect or adherence in Tai Chi Chuan. Chan and lien are vertical adhering movements, lifting from above and supporting from below, respectively. Tieh is adherence in the horizontal motion, sui is adherence from the rear. Pu tiu pu ting means neither to lose the adherence nor to resist.
Reference: T’Ai Chi Ch’Uan Ta Wen, Questions and Answers on T’Ai Chi Boxing Chen Wei-Ming ( Benjamin Pang Jeng Lo & Robert W. Smith ) North Atlantic Books 1985