Consider the upper arms

In the left arm, a firm bicep and relaxation from the elbow to wrist helps the fingers to be light and the vibrato be free (yet controlled.) Similarly, in the right arm, a firm bicep and relaxation from the raised elbow (importantly held above the string level) to the wrist, allows the bow pressure to emanate from the first finger - a laser of pressure opposed to a butcher knife. (Note: remember to adequately pronate the bow arm inward - toward the point of pressure)