Col Legno means to play with the wood of the bow and is not as new a technique as some other extended techniques. It does however change the timbre that the violin can produce as the performer is not using the hair of the bow that produces a clean sound. The sound created by Col Legno is much more faint and whispery.
Col Legno Butatto means to bounce the wood of the bow and creates a much more percussive sound, this is used by Chopin in Piano Concerto no. 2 in the final movement. It gives a feel of wood being struck together sticking with the peasant dance theme that he had throughout the final movement.
If it is to be bowed then Col Legno Tratto is to be used, its gives a whispery white noise effect. Crumb calls for this in Black Angels which is used in a section called ‘Sounds of Bones and Flutes’ using this technique gives it that haunting feeling that you would expect from bones and flutes.
Care must be taken when writing Col Legno as this can be a destructive technique towards the bow, it may chip or scratch the varnish on the bow and serious players could have an extremely expensive one. For violinists that are thinking about performing extended techniques a lot then they should think about having a cheaper spare bow.
When playing this technique I found that it was easier to have my thumb underneath the leather with the other fingers on top, as you may have done when you were first learning to play the violin. I found that it gave much more control and when playing Butatto it gives a good pivot point to help keep a striking rhythm.