a BOV vents off built up pressure when throttle is cut off. this
prolongs the lifespan of the turbocharger by reducing the resultant
pressure on the turbine fins, but it is not necessary for low or
stock boost conditions. and it certainly does nothing to reduce
turbo lag. if installing a few hundred dollar cheapo device can
reduce turbo lag, why should ppl spend thousands of dollars on an
anti lag management system? 
for most part of it, a poorly matched or improperly tuned BOV will
wreck havoc on the MAP sensor readings in a turbocharged vehicle. u
are lucky the evo is less susceptible to such probs. usually, its
young punks who wanna show off and know nuts about BOVs that
install them. high powered evos producing in excess of 700 hps are
all running waste gates instead of ricey BOVs. 
