My professor calls this a tear drop motion. Up bow, hand moves inward towards your face. Down bow, hand moves outwards from face. The shape you make with your hand becomes a subtle teardrop that keeps the string vibrating at each bow change. The tip will change in angle very subtlety. It will be difficult to learn but that’s the concept I believe you are looking for. Practice with long bowing and gradually shorten the bow usage for each tear drop to make it 2nd nature.