How did Herbert Spencer connect laughter with released nervous energy?

In the context of human behavior, the 19th-century English philosopher Herbert Spencer formulated a physiological explanation for laughter based on the concept of "liberated nerve force."

Spencer proposed that sensations and emotions naturally generate bodily movements. He observed that when consciousness is suddenly shifted from significant matters to trivial ones, the energy that was previously directed toward the "great" subject becomes redundant. This "liberated nerve force" must then be expended, and it does so by following the physiological channels of least resistance, which manifest as the bodily movements of laughter.1

Essentially, Spencer viewed laughter as a safety valve for surplus tension. This theory was later incorporated by Sigmund Freud, who emphasized the release of repressed emotions and suggested that the specific physical expressions of laughter—such as facial contortions—evolved from primal, pleasurable processes of detensioning.1