Part of standard string technique includes the use of a left hand vibrato.
It is possible to vary the degree of vibrato from senza to molto. Composers have also graphically the degree of vibrato in terms of speed and/or degree of fluctuation in pitch. If no such marking is given to the contrary then a normal vibrato will generally be assumed.
The video demonstrates a gradual change from a slow, wide vibrato to a rapid, narrow vibrato.
Graphic notation should correspond to the desired result.
1. Vibrato of various depths and speed.
2. Progressively wider and faster vibrato.