Just found this with a quick google search, explains a lot better than I did on what the BOSS really does.
http://www.hk94.com/hk/topic/7738-winchester-boss-system/
How the BOSS™ Works
Barrel vibrations are the primary cause of rifle inaccuracy. Every time a rifle is fired, barrel vibrations are set in motion. These vibrations occur in all directions, even back and forth along the axis of the barrel. They are influenced by many factors such as bullet type, bullet weight, primers, different powders and even barrel configurations.
The BOSS (Ballistic Optimizing Shooting System) is an adjustable weight that alters rifle barrel vibrations. By adjusting the weight on the BOSS, you can tune your rifle to specific ammunition, allowing a bullet to consistently exit the rifle muzzle at the optimum point in barrel vibration. At this optimum point, either at the peak or valley of a vibration wave, the rifle barrel is momentarily motionless. The suggested “sweet spots” (found on a separate sheet supplied with this manual) are the starting points for determining the optimum accuracy of your BOSS-equipped Model 70. Once the true “sweet spot” is determined for a given ammunition, your rifle will consistently deliver extraordinarily tight groups. The BOSS allows you to tune your rifle to your ammunition to get the optimum accuracy for all available factory loads.
When adjusted to the “sweet spot,” the BOSS will compensate for slight deviations in ammunition velocity. The BOSS will not, however, compensate for poor shooting or flyers caused by low quality bullets and ammunition.
To obtain the highest level of accuracy from your rifle it is recommended that the rifle bore be cleaned every 25 to 30 rounds. See “cleaning the rifle bore” section of this booklet for cleaning procedures.
Muzzle Brake Feature
The unique exhaust hole pattern on the BOSS provides considerable recoil reduction. Recoil reduction of the BOSS ranges from 30% to 50% — depending on caliber, weight of rifle, etc.
You will experience an increase in noise and concussion (muzzle blast) when a BOSS equipped rifle is fired. Hearing protectors (shooting ear plugs or muffs) should always be worn to guard against hearing damage when shooting a BOSS equipped rifle. Wear shooting glasses to protect your eyes from flying particles. Always keep a safe distance between the muzzle of your firearm and any persons nearby, as muzzle blast, debris and ejecting shells could inflict serious injury.