The finger tip that is pressing on the string should be rolling so as to change the length of the string between the figure and the saddle. Another type of vibrato is by varying the pressure of the finger tip onto the string to change the tension of the string. In practice, it is always the combination of the two. But different combination of these 2 have different tone or color. Try different combination to get the tone you wanted to suit different requirement. Good luck and enjoy your erhu.
