Hi, - I want to improve my vibrato. On down bow it's OK - on up bow however is still feels unnatural and I can't last it too long... I feel uncoordinated ... I have seen the youtube vids (Klinghoffer etc., various cello vids) to check my technique. My thumb is behind my second finger at the back of the neck, when second finger id doing vibrato. Somehow, I cannot succeed to get a nice vibrato like on up bow. Any tips?