First, the original question has been answered. Our system is closed and ALL the air that travels through the MAF goes through the engine. Any kind of "blow-off" valve will mess up the air/fuel mixture.
Our "cool factor" is blower "whine"
. FWIW, there are mods that enhance this sound quite nicely
Second, supercharger operation in an SC is often misunderstood. Since it is driven from the crank, its speed is directly relative to engine RPM. This means that it always moves (pumps) the maximum amount of air it can for any given inlet pressure and RPM. Our blowers are a "positive displacement" type. They do not "compress" the air charge; they move separate "parcels" of air through the case, and they do it by force (hence the term "positive displacement"). Compression (boost) is the result of all those "parcels" getting squeezed into the intake tract.
Constantly boosting* air requires extra HP to turn the blower. At idle and light cruise we really don't want to waste power trying to boost* the intake charge (because we don't need to), so a bypass valve was designed into the system. This allows a portion of the boosted* air from the supercharger output to "re-circulate" back into the supercharger inlet through the bypass valve. In effect this "equalizes" the inlet and output pressures and removes the supercharger load from boosting*. This greatly reduces the amount of power required to turn the blower.
Ford measured the HP required to turn a blower with the bypass valve fully open to be roughly 1/2HP; without it about 4HP. Since the blower is "in effect" free-wheeling, blower wear is also reduced. One last benefit of the bypass valve is improved "driveability".
*Note - Boost in this case is the output pressure relative to the inlet pressure of the supercharger.