Multiphonics are a phenomenon usually reserved for brass and wind instruments. It occurs when it is possible to alter the way of blowing to produce two or more tones at the same time. With the violin this is obviously possible by performing double stopping but it is also possible to perform a real multiphonic producing two notes whilst only bowing one string. It gives a slightly distorted sound and is the breaking up of the various other partials.
This seems to be a relatively new technique as such I was not able to find notation for it, although Patricia Strange suggests a possible notation for it.
Tracy Silverman describes how to play it:
I found this a little hard to understand but the way I managed to do it was to play the harmonic in 3rd position and to slowly apply pressure with your finger until you hear the E at ‘normal’ pitch as well. I found that if you keep firm bow pressure it helps to keep a consistent tone.
It is important for the composers to work closely with the performers as the possible multiphonics will differ from performer to performer and instrument to instrument.