Vibrato is use the left hand's movement to make the string vibrationg and to produce voice like beautiful sound. Vibrato is very important technich for the Erhu. By doing the vibrator, need to form the left hand as a round shape which feels that as if can insert an egg inside of your Palm. While you are vibrating, the only contact of you hand to the neck of the erhu is the " Inner side of the second knuckle" of you left thumb. As you put your first or second finger on the string with the knuckles bend,( The first knuck of the fingers must bend). then the view from the top would look like a circle. So at this moment, the inner side of your the knuckles of the fingers has no touch at all with the neck of the erhu. Then you start to roll with the hand totally relaxed from the wrist down, by doing so, the first knuckle of the finger which on the string, will be " bend and straight"( but this would be be very very slow motion, I suggest that you concentrate on looking at the all hand to see if its relaxed or not. You need to think that your finger which on the string is "massaging the string, and to imagine it's like the base of the baby "rock chair."