Piston engines produce pressure in the crankcase (basically the lower half if you picture from the crankshaft down) as the pistons pump up and down. If that pressure were to build it can reduce ring seal, hence the purpose of the ventilation system.
Because of emissions reasons, however, that pressure cannot be vented to the atmosphere, so the manufacturers have to route it into the intake manifold so that it can be burned in the combustion chamber. The gasses from the crankcase carry emulsified oil that coats the intake runners (producing that sludge you see when you remove the manifold on a high-mileage engine) and degrades the octane of the atomized fuel entering the combustion chamber. In other words, it's killing the octane rating on that big tank of premium you just paid for.
WARNING: This product is not street legal and may only be used on Racing Vehicles. Racing Vehicles must be used exclusively for racing or other forms of competition. Racing vehicles must not be registered and must never be used on the street. It is illegal to install this product on a registered vehicle and use it on the street.