The drone playing is similar to the finger-picking in guitar playing. In finger-picking we try to play simultaneously, the melody and the underlying chords. In drone-playing the underlying chord is only one, note, the 1st or 5th of the underlying chord, or both. And these 1 or 5 are essentially open strings up or down to the string that we play the part in the current tetra-chord) of the melody.
See e.g.
