This is quite a delicate technique and evolves from guitar and banjo technique. The difference between the fingerboard on the violin and the banjo is obviously the fact that the banjo has frets, meaning that the sound of the L.H. can be heard much more easily. On the violin this is a much more delicate technique one which I feel that could benefit from amplification or would be extremely effective as an orchestral tool.
I found that hammering works best on the G and D strings as they are naturally louder and thicker meaning that it is possible to trap the string between the fingerboard and your finger. It gives a very percussive sound to the violin.