spiccato, a delicate balance between controlling the bow and losing any tension and letting it bounce. It should be named the zen of violin bowing. i have even seen videos where the violinist actually lifts the 3rd and 4th fingers off the stick to let it bounce more. The stroke itself isn’t so difficult, but the literature where it is used is usually relatively quick tempo, with tricky fingering, so for me, it’s the coordination of the bounce with the l.h. finger movements. aaahhhh!