 Different placements of this tube along the string influence or eliminate the frequency at which the wolf occurs.It is essentially an attenuator that slightly shifts the natural frequency of the string (and/or instrument body) cutting down on the reverberation.An older device on cellos was a fifth string that could be tuned to the wolf frequency; fingering an octave above or below also attenuates the effect somewhat, as does the trick of squeezing with the knees.