This pizzicato exercise is useful to free up the right arm and to practise the circular movement of a down-bow retake.
Use an open hand for the pizzicato and aim to stroke at least two strings. This will encourage a release in both the hand and in the arm movement.
When you add the bow work towards maintaining that open feeling in the hand. Landing on the string without hesitation is a good way to monitor 'give' in the bow hand. Listen for a ringing sound that continues after the bow leaves the string.
Finally, set the bow on the string before playing. Use the momentary pause to check that the bow hand is released.
Down-bow retakes can use a stroke that is placed on the string before playing, or a stroke that comes from the air. In both cases the resonance comes from bow speed, without any additional pressure from the bow hand. Use enough bow, since the speed gives the energy for the retake action. We need to think of the retake as being one cycle of action and rebound, much like vibrato has its action and rebound....