Hand & Mouth

Grab a harmonica... hold it... play it... now pull the harp away from your mouth and look at the index finger of your holding hand. The farther back you place that index finger, the more room your mouth/lips have to place the harmonica deep in your mouth. The deeper the harmonica is in your mouth (within reason of course) the bigger the tone.

For more detailed information watch the Lesson "Blues Harmonica Fundamentals" and go to "Getting Started - Blues Harmonica Fundamentals" and watch the video "Holding the Harmonica" @ 1:15