Wikipedia describes how the sitar is able to produce the sounds and overtones:
The sitar's curved frets are movable, allowing fine tuning, and raised so that sympathetic strings (tarbs, also known as "tarif" or "tarifdar") can run underneath them. A sitar can have 21, 22 or 23 strings, among them six or seven played strings which run over the frets: the Gandhar-pancham sitar (used by Vilayat Khan and his disciples) has six playable strings, whereas the Kharaj-pancham sitar, used in the Maihaar gharana (Ravi Shankar), has seven. Three of these (or four on a Kharaj-pancham sitar), called the chikari, simply provide a drone: the rest are used to play the melody, though the first string (baj tar) is most used.
The instrument has two bridges; the large bridge (bada goraj) for the playing and drone strings and the small bridge (chota goraj) for the sympathetic strings. Its timbre results from the way the strings interact with the wide, sloping bridge. As a string reverberates its length changes slightly as its edge touches the bridge, promoting the creation of overtones and giving the sound its distinctive, tone. The maintenance of this specific tone by shaping the bridge is called jawari. Many musicians rely on instrument makers to adjust this.We only heard it played for a few enchanting minutes. I am tempted to look up music from Ravi Shankar and find music from other masters. Would it be enjoyable to listen to for a whole CD?