Crumb Black Angel (1970)This technique can be used to create either a timberal effect or a pitched effect. Depending on the distance of the bow to the bridge it can change the effect, the farther away from the bridge more tone will be produced.
The tone of this technique reminds me of n overdrive pedal on an electric guitar, it also works extremely well with overpressure. The closer to the bridge will result in higher harmonics being produced. If a constant dynamic is required then more bow pressure will be needed as the bow reaches the tip.
Playing on the bridge will result in a timberal effect as the bow hair will be divided across the bridge and it will also be playing the actual wood of the bridge. For example William Sydeman writes a chord to be played on the bridge and indicates that specific pitches are not wanted. This is depicted with the note heads looking more like drum notation than a normal classical notation. For shorter more sticatto sound it might be needed to dampen the strings by clamping the L.H. to the fingerboard.