it has a pseudo effect- it sounds better(more like a BOV), but performs worse. A few swear by it, but noone can show any hard #s to prove it. And if you think about it, it doesn't make any sense, basically a DV is a Tvalve and a spring loaded to keep the diaphragm closed until a certain pressure is achieved. It just can't possibly work the right way.
For what it's worth a friend of mine was trying to run his at the track reversed, it definatelty sounded more like a BOV but it was killing his time by almost a full second. And I don't know if it is connected or not but his ECU died a week later.