Q: What is overblowing and why may I want to do it?

A: With bends you can lower the pitch of the draw notes in holes 1-6 and
the blow notes in holes 7-10. With overblows you can raise the pitch of the
blow notes in holes 1-6 and of the draw notes in holes 7-10. These latter
are actually called ‘overdraws’. Overblows and overdraws are much harder
to achieve then bends, but can give you the accidentals and chromatic notes
not usually available on a diatonic. A good way to practice is to take of
the covers of your harmonica, blow through hole and stop the *blow* reed
from moving with your finger. Now try to make the draw reed sound. This
gives you an idea of how an overblow should feel. Now try to achieve the
same without your finger. The other approach to chromatic scales on a
diatonic is to valve the draw reeds in holes 1-6 and the blow reeds in
holes 7-10. This way you can bend the otherwise un-bendable notes, while
you can also still make the ordinary bends. Note that playing on a valved
diatonic might seem easier than learning overblows, but it is said to be
almost as hard to get good tone and control in that way — BB/HAA