When a composer or string arranger wishes to create a specifically warm or hazy sound, they may well write in the direction ‘Sul Tasto’ meaning ‘on the touch or on the fingerboard’. In effect this means bringing the bow further away from the bridge until it is over the end of the finger board. Here, the string has less tension as it is further from the point of suspension on the bridge and therefore has less resistance, so cannot take as much pressure. Generally Sul Tasto is used for a softer dynamic (such as p or pp) and like a harmonic, a faster, gentler bow is utilised.
Yehudi Menuhin defines it as giving “….a velvety and cooing sound. The string is soft, not as resistant, and cannot take any pressure….”
When a full string section plays Sul Tasto, the effect can be of a very soft sheen with any surface noise absorbed by having so many players. In a solo instrument, recording with a close microphone can pick up some of the surface noise (the bow against the string), but it has a lilting, ethereal quality, not unlike a flute.