Bow pressure Graphic
Overpressure is one of the most commonly used extended techniques. It creates a loud scratchy white noise sound, which can be very effective to create a completely different sound. It is not a sound usually wanted from a violin as the sound is usually a sign of someone who has not developed proper technique as violinists usually strive to create a pleasant tone.
Overpressure can be used in two ways, one where the strings are dampened and the other where a pitch is wanted to be heard by the composer. To dampen the strings the violinist simply clamps their L.H. on to the strings restricting their ability to vibrate properly. To create overpressure the violinist has to, as the name suggests, create large amounts of pressure. I found that the best way was to have a heavy right arm and really pull threw the bow stroke whilst pinching and pressing hard with the first finger on the R.H. remembering that if constant overpressure is wanted then more pressure will be needed nearer the tip of the bow.
Crumb Black Angels (1970)
There are various different symbols for overpressure to be notated with. Crumb uses this technique in Black Angels, starting with a normal note at pp increasing in volume and pressure depicted by the zigzag line to fff. As a composer you would have to realise that this technique can only really be played loud as the more pressure created on the string then the more louder the volume created. Another way to depict differing amounts of overpressure is to create a graphic above the stave as in Rounds by David Ernst. Although written for cello the theory is still the same. The bigger the graphic the more pressure needed.