Dounis always told his students to always imagine they were playing on a string 1/2 an inch above the real string.
To understand what he meant, do this exercise.
While holding the bow, put your arm at your side. Then close your eyes and bring the bow to a 1/2 an inch above the string. Now, without moving your arm, reach down with the hand and play a note. When you finish the hand should spring back to its original position.
This principle is intimately related to the idea of the curve...in otherwords the bow is always moving in a curve (or semicircle, circle or arc.). If mastered it gives the bow arm greater freedom and flexibility.