Cross-String Bowing - Tremolando

For this effect, use a strumming-like wrist action (which is much faster than arm movement) to bow back and forth between two strings rapidly. This also works great over one string, while muting the adjacent strings.

Note: Though the highest notes on the high E string (above the 17th fret or so) may not sustain, you can play spiccato, tremolando and arpeggios all the way up to the highest fret. Keep in mind that the harmonic mode drives the high notes more easily.