Nothing. But PPV is generally behind the browser just fyi.

I've had plenty of success with PPV, and so have others.

Pop up ads are what leadimpact offers for advertising so it's essentially the same thing

Pop up is on top of the browser, pop under is behind the browser. Lead impact does that and it's called ppv marketing. (Cpv is the industry term for it but PPV was invented as a term by a guy who thinks he invented the traffic source).