Conrad,
There are two components that create recoil: the acceleration of the bullet, and the acceleration of powder/gas (the powder creates the gas as it burns).
The recoil caused by the bullet being accelerated down the bore cannot be eliminated so lets look at the acceleration of the gases.
As the powder burns, the gas forces the bullet to the end of the barrel. The mass of the powder has turned into a gas and has expanded or accelerated down the barrel and is pushing on the base of the bullet. This part of the acceleration of the gas cannot be eliminated.....it's required to push the bullet down the barrel.
Now the part that can be reduced. When the bullet exits the bore, the gas (which has been under significant pressure) now has an opening to escape. The gas then accelerates down the barrel to exit the muzzle. The exit velocity of the gas can be quite high. If the mass of the gas is roughly equal to the mass of the powder and it is accelerated to speeds greater than the bullet, then it produces a significant amout of recoil. If the gases are accelerating forward (down the barrel) recoil is in the opposite direction (toward your shoulder).
What if you changed the direction some of the gas was venting? Wouldn't the redirected gas accelerate in the new direction and that gas would no longer be pushing against your shoulder? If you change the direcion of the exiting gas where it angles back slightly toward the shooter, not only will the redirected gas not push against the shooters shoulder but will actually work to push the rifle away from the shooters shoulder thus cancelling some of the effect of gas that does accelerate out the end of the muzzle.
That is how a muzzle break works.....by changing the direction that some of the gas is accelerated it pushes (recoils) in a new direction. If you vent gas out opposite sides of a muzzle break the recoil from each side cancels the other out (and neither is pushing toward the shooters shoulder).
If you redirect some of that high pressure gas in the direction of the shooter, imagine what a pressure wave hits the shooters eardrum.
HogWild
