The strings are strung across a snare bridge, which creates the warm buzzing sound full of overtones which has been the hallmark of Indian music for centuries.
In classical Indian music the accompaniment instruments have no chord changes. The root note remains constant throughout the piece and provides a background for the melody instrument. The melody instrument can be a violin, a flute, vocals, a santur or another instrument.
As there are no harmonic changes in the accompaniment, it is much simpler for the melody players to improvise freely, because they do not have to consider chord changes in the flow of their melody.