U-Blocking Defined

U-blocking on the harmonica is where air travels down the center of the
tongue... commonly where the left side of the tongue blocks the hole to the
left and the right side of the tongue blocks the hole to the right (most
players will have their lips over three holes when doing this, e.g., when
playing hole 2 your tongue would block holes 1 and 3). The tongue is either
curled in a very evident "U" shape (this is also genetics at play here...
not everyone can do this) or there's much less of a prominent U-shape and
more of a scoop or valley, where the air travels down the center (this is
most commonly the case when players aren't aware that they're doing it).

This technique is not a bad technique, it just causes problems when using
traditional tongue block techniques. When you perform a standard tongue
block, your lips are over four holes (1-4 for example) and your tongue
blocks holes 1-3, leaving the 4th hole to sound...

Slap
Achieved by starting with your tongue off of the harmonica, sounding a
chord, all of which is LOWER than the final 4 draw note (except of course
the 4 draw itself) and then slapping down with light pressure over holes
1-3. This creates a chordal introduction to the 4 draw, with all of the
notes swooping up, like a glissando (gliss) to the higher note (4 draw)

Pull
Starts with the tongue on the harp, blocking all four holes (1-4) and then
pulls off to sound the chord. The pull and slap techniques are commonly
paired, making it important that the lips are over the fours holes (1-4) so
that no mouth movement is necessary between the two techniques. In a
U-block, the lips would be over holes 3-5, sounding the 4 draw in the
center... then if you want to do a pull, you would need to move the lips
over to holes 1-4. You can do a pull on holes 3-5, where the lips are for
the U-block, but the resulting chord is a little thinner (and so is the slap
if it's performed in this way).

Flutter
Same as the slap, just repeated.

Octave
This is where the rubber meets the road in regards to the use of the U-block.
Having the lips over holes 1-4, you can slap (1-4 to 4), flutter (alternating 1-4 to 4),
pull (block to 1-4), octave switch (4 to 1 to 4, etc.), or octave (1 and 4 together). In
all actions the mouth is over four holes, IT NEVER NEEDS TO MOVE... VERY
EFFICIENT. To perform the octave from the U-block requires a large movement,
from 3-5, to 1-4. Keep in mind that these tongue block techniques are used
in combinations at very fast speeds, which makes it important to stay in the
1-4 embouchure.

Final Thoughts
In the end, it's not a crime to U-block... you can use all of the techniques
I brought up. Your slaps and pulls will be a little thinner sounding (it works
for John Nemeth, and he sounds great) and you won't be able to use octaves
as instinctively as in the tradition tongue blocking embouchure... you'll
end up using octaves in a run and then return back to U-blocking... in other
words you'll need to make a switch and then switch back, in the same way
that the pucker player moves back and forth from puckering to tongue
blocking.

Why Isn't the U-Block Taught as a Technique?
There are no advantages to the embouchure. U-blocking also commonly causes problems
when someone wants to learn how to perform standard tongue block techniques.

In the end...
Nobody cares which embouchure you use... they only care that you create
great music. I also don't care which one you CHOOSE, I only care
that you're aware of the pros and cons. My job is to teach you how to most
efficiently use technique in our world of masterful harmonica playing.

Pictures
Visit http://www.facebook.com/bluesharmonica for pictures of what I'm speaking of