The duration of stopped string vibration increases with contact time between finger and string because the vibration can be reflected again and again at the stopping finger, until the excitation energy has been exhausted. The situation regarding harmonics is the opposite. Since vibration passes the touching finger, and in doing so always loses a certain amount of energy through damping, duration is maximised when contact between finger and string is as short as possible. However, there is a minimum contact time that allows the harmonic to vibrate at a stable pitch. This is usually very short, but is relative to the frequency of the harmonic. If the finger is removed from the string before this, it is likely that other neighbouring harmonics and especially the open string will also be present in the sound.
The decay of a particular sound is maximised if reflection of the kink in the vibrating string is allowed to continue until it runs out of energy, i.e., if the string is allowed to ‘spend’ all of its excitation energy. If the stopping finger is removed from the string during a tone’s decay this process is interrupted, vibration may continue for a short while as the vibration decays in the cello body and the room. Since these effects are weak compared to the vibrating string being radiated to the body/room, removing the stopping finger can be an abrupt damping of sound.
In the case of harmonics, the optimal contact time between touching finger and string, usually very short, is the time taken for the string to vibrate for a few wavelengths at the particular harmonic’s frequency. In this time the string begins to vibrate stably at the harmonic pitch and continued contact with the left hand serves only to dampen vibration because, since vibration is reflected at the nut rather than the left hand finger, the kink of vibration passes the touching finger with every cycle, loosing energy in the process through damping. Since contact time is dependent on wavelength, optimal contact time reduces for ascending harmonics.