I first learned this in the mid 1980's (showing my age here) during basic SWAT training.  
For those who aren't aware of it, the process involves shooting at a partially concealed or covered target by deflecting your shoot off a hard surface at a shallow angle, so that it travels parallel to the surface.
We first learned it for shooting at a subject standing behind a car with his feet/ankles exposed when you're returning fire from the prone or kneeling position and due to angles or risk areas either cannot align with the target or risk too much exposure.
We also trained to do this off a hard wall surface, as that many of the areas we were responsible for had hard concrete walls.
It can be a helpful little skill to have in your bag of tricks.
Anybody else train and practice this way?

Learned that little trick from a former Delta guy at BSR.  He called it "rabbiting" (sp?)

It underscores the use of a vehicle as concealment, as opposed to cover!

Ken -

Yep, same thing (and possiably the same trainer).  I've heard it called "rabbitting" or "skimming".  
We practice it with short lenghts of 2x4's standing upright behind a car on a part of the hard pad.