The term refers to a situation in which people understand each other without the need to utter a single word. It generally means the spontaneous understanding reached by close friends who share common interests, aspirations, and dispositions. In particular, it refers to an aesthetic state in which the subject and the object interact with each other smoothly with no barrier between them, or in which an artist creates a marvelous image and a viewer appreciates it with emotion and understanding. The culmination of such an experience is joy and satisfaction derived from the perfect harmony between the human heart and its surroundings.
When Emperor Jianwen of Liang in the Southern Dynasties was touring the Hualin Garden, he turned to his followers and said, “A place which prompts heart-to-heart communication need not be far. This garden is shadowed by trees and has a stream meandering through. Such a place makes one think of Zhuangzi strolling on the bridge of the Haoshui River and angling in the Pushui River, where birds and fish seemed eager to get close to him.” (Liu Yiqing: A New Account of Tales of the World)
The Book of Songs contains odes, satires, and admonitions, but all are veiled. One must engage in a heart-to-heart communication to appreciate them. (Jiang Kui: The Poetry Theory of Baishi Daoren)