If you listen to the shamisen very closely, you will hear a slight buzz coming from the top (lowest sounding) string.  Usually tuned D G D or D A D, the string resonates anytime the player plays a note within the overtone series.

So if your top string is D, then ascending A, D(octave), F#, A, C, D, E, F#.

Notes like F# have a major quality to them, and if our root string is tuned to D, then most likely the key will be something like a western G minor pentatonic with a 4th and flat 7 slipped in here and there. The root is the 5th.

D Eb (F) G A Bb (C) D

The following piece has more E natural than Eb in it.

See if you can hear the drone like sound in the shamisen.