usage

The back of the tongue is used for articulation with all embouchures (it's the primary articulation for tongue blocking). This is commonly accessed by using "g," as in "ga" or "k," as in "ka." When bending this turns into "gu" and "ku." This is also the area of the tongue that's raised when bending lower-pitched reeds, like 3, 2 and 1 on an A Harmonica for example.

Behind the blade of the tongue (what we'll call the middle-front) is the area used to control draw bends on holes 5 and 6, and blow bends on holes 7 through 10. This area of the tongue requires the most training and time to develop. The middle or back of the tongue is easy to raise and interact with the roof of the mouth. The middle-front has the challenge of working in the area of the mouth where there's a sharp curvature, between the Post-alveolar and Pre-palatal (location 5 and 6 in the diagram here http://en.wikipedia.org/wiki/Alveolar_consonants). continue reading...

When watching players like Sonny Terry and Rice Miller on video you'll sometimes see them move their hands left to right or fan the harmonica while playing. Though they don't touch the harmonica, this rhythmic movement in front of the harmonica does alter their tone and volume in a rhythmic way. It's hard to call this a full-on tremolo due to its short duration and small tonal change, but it's cool and worthy of mention.