It DOES reduce total recoil, as explained by 2-Piper.

Recoil is produced by the mass and velocity of the total ejecta acting along the bore line in accord with Newton's third law.

When you vent off some gas sideways, you reduce recoil proportional to the mass vented.

This can be calculated, measured, and felt.

http://en.wikipedia.org/wiki/Muzzle_brake


"The price of good shotgunnery is constant practice" - Fred Kimble