The term "release"
in rehabilitation, performance, and manual therapy settings, generally refers to the feeling of a reduction in tissue tone, tension, density or activity in response to pressure, position, vibration, mobilization or manipulation of soft tissue. The mechanism that results in a feeling of "release" is likely a complex set of mechanical changes and neuromuscular reflexes (example, depicted below in the Schleip Model of Tissue Manipulation); however, it assumed that "autogenic inhibition" plays the largest role, as a result of stimulating golgi tendon organ, ruffini endings and/or pacinian corpuscles.
Tissue Manipulation Model - Schleip, R. (2003). Fascial plasticity–a new neurobiological explanation Part 2. Journal of Bodywork and movement therapies, 7(2), 104-116.