Develop finger, thumb and hand flexibility with a string crossing study like Fiorillos "Caprice No.29".
Keep the upper arm on the middle string and make your hand and fingers play the détaché stroke. Also do the string crossing over three strings with hand and finger action only. Notice that the fingers use collé action.
Kreutzers "Study No.13" can also be used for this exercise.
Beginning Clicks - Introduction to Collé
Resources - Repertoire