In Zhuangzi, the chapter entitled Dazongshi 大宗師 (The Great and Venerable Teacher) describes a perfected being (zhenren 真人):
古之真人，其寢不夢，其覺無憂，其食不甘，其息深深。真人之息以 踵，眾人之息以喉。
The perfected of old slept without dreaming and woke without worrying. They ate without delighting in taste and breathed deeply. In fact the perfected breathed all the way to their heels, unlike ordinary folk who breathe only as far as their throats.
Although these perfected beings may not have engaged in daoyin, their ability to ‘breathe all the way to their heels’ was an inspirational concept for those who were engaging themselves with breathing and daoyin exercises. The technique which is based on Zhuanzi’s concept of breathing to the heels can be found in Yunji qiqian, j. 58, Fu yuanqi fa 服元氣法 (The Method of Ingesting the Original Qi).