Think of it this way. The throttle body is open when you are full throttle or revving it like in the video. The turbo is pumping a bunch of air through it. You let off the throttle to shift or what ever and the throttle body closes. The turbo is still pumping air, it has to go somewhere, so that is where the blow off valve or bypass valve comes in. I might have this screwed up, but I think a blow off valve just vents the excess air to the atmosphere, where a bypass valve routes it back to a point ahead of the turbo but behind the MAF. Our cars need a bypass valve because venting all that metered air out would cause it to confuse the computer. At least I know the Vortech has a bypass valve for this reason.
Hope this helps
