What we do is actually improvise over a chord that is not there, it sounds crazy but thats the idea, so here we improvise over an A minor chord but for the Super imposing part we use a D Pentatonic scale giving us the flat sixth. 

Now you might say that we get that note anyway if we use a natural minor or Harmonic minor or Melodic minor scale. But if we want to keepour Pentatonic sound and also in this case our Pentatonic chops we do it this way, see the video to get a better idea