The so-called "lianlian" technique is "when two items A and B are spoken in succession, the words that are applicable to item A are used to express the concept of B" (Chen Wangdao's "Rhetoric Revealed") 》).
“If the heavy door is not locked, the dream of lovesickness can wander around the world at will.” (Zhao Lingji’s "Crying of the Night·Spring Thoughts") The word "lock" connects "heavy door" and "dream" together. It describes the protagonist's feelings of being lonely, bored, depressed and depressed.