Powerful high end

There are three levels, via technique, of how powerfully you can play the high-end of the harmonica.

1) Single notes with conviction
2) Slaps, which brings in the texture of the preceding chord = more powerful
3) Octaves, which supports each note played with its octave below brother = most powerful

So, when playing a heavy, powerful blues solo on the lower six holes and you want to go to the high end, octaves are your best bet to match that intensity.