The purpose of a BOV is to release pressure build up (once the turbo is spooling/ spinning) in the intake pipe when your foot lifts off the gas and the throttle body closes.

When this happens, and the turbo is spooling, pressure builds up between the compressor wheel of the turbo and the throttle body as the pressure has no where to go and results in slowing of the compressor wheel (the speed at which the turbo is spinning). A bov, releases this pressure and allows the turbo to free spin so when you get back on the gas there is minimal lag between hitting top end boost.

All turbo cars will come fitted from factory with a bi-pass valve, this is essentially a BOV which does not vent to atmosphere (giving the whoosh sound) and so when the valve opens the air pressure is released from the intake pipe allowing the turbo to continue spinning though air is diverted back into the closed intake system.

Advantages of an after market bov is their adjust-ability which you won't get from a standard unit and the noise if you want it. Ajustability is not so much necessary on a standard turbo setup as the factory valve is designed around the boost capability of the turbo, its only when you start slapping on big boy turbo's beyond the capability of the factory units you would need this.

Disadvantages are that they will richen up your AF ratios as everytime it opens you are losing registered intake air, with excess fuel being applied to a reduced air intake volume.

By increasing spring tension, which you can do by adjusting the bov, like the one in the video, you are essentially stopping the BOV from opening, and the flutter sound you hear is the air bouncing off the closed throttle body butterfly, back into the spinning compressor wheel. If you want this sound (which I think sounds wicked) simply get rid of your BOV altogether for the same desired effect, the disadvantage to this is that you will get greater lag and increased spool time between getting off and back on the gas.

If you want the best of all worlds, the sound without decreased performance, go the antilag route like the rally cars do... though this is a sure way to decreased turbo life.

Hope this helps