I learned to do this back in 1986.  It had been used in gunfights by the L.A. Sheriff's Deputies several times and a viable tactic.  This was the age before lawyers wrote qualification courses and Internal Affairs guys were in charge of tactical training.  Before the silly statement was spoken in fear, "every bullet has a lawyer attached to it", and the death before litigation culture became the norm.

Today, I don’t know of any private or public sector schools teaching this actively.  I know that we teach it and make it a common activity in our shotgun classes unless the surface simply doesn’t allow for it.

The discussion is on using the ricochet effect, particularly with buckshot, to our tactical benefit.  In the old days before tactical-speak, it was called “skip shooting”.  It can be used with any projectile to a degree, but I learned it, and it is easiest to pull off, with buckshot.

Buckshot pellets are not "soft and bouncy" like a rubber ball, so most of the momentum perpendicular to the surface is lost and the projectile leaves the impact surface at a very shallow angle.  The angle you hit the surface with is not critical since the projectiles will leave the impact point at an angle almost parallel to the surface, and at a slightly increasing angle.  I wrote about this effect  in Tactical Advantage with regard to using cover.

So here is where it fits in the skill matrix.  Ricochet shooting, or "skipping the rounds" is used to shoot around cover, or to shoot underneath cover.

If an adversary is shooting at you from cover of a concrete or hard brick wall, and you cannot maneuver for a clean direct shot, you can hit the wall, at a shallow angle reasonably close to his position.  This will yield some sort of hit on the adversary.  As the surface you are attempting to skip the rounds off becomes softer there will be more penetration, particularly as the angle increases, thus the result will be degraded. 

Any urban area such as a street or a parking lot will show the adversary's position (legs and feet) behind a car by the diffusion of light, as you look toward the lower area of the car.  You don't need to drop down into a prone to see the feet but simply aim at the reflection under the car and skip the buckshot at their feet.  This falls under the category of "diminishing the adversary".  Certainly a shot at the feet will not kill them, but it will open the door for more carefully placed gunfire. It was not discussed tactically but I believe LAPD SWAT used this method to shoot Mătăsăreanu under the vehicle he was using for cover.

If you are shooting at sheet metal, such as a motor vehicle side panel, at shallow angles the bullet will skip, or ricochet as expected.  As the angle increases and is made steeper, at some point, the projectile will penetrate the surface. The same is true of windshield.

Here is an old FBI training video that gives a clear idea of the dynamics of Skiping Buckshot and its tactical applications. Most relevant portions are from 3:36 to 6:35.