What if visitors have popup killers or if they disable javascript?
Before displaying campaigns in the popunder windows, and before using your credits for this, we display a test campaign, with encrypted data. If this test campaign is loaded, the encrypted data is verified, in order to avoid abuse. If everything matches, then it`s certain that the visitor has a window open under his current window. So in this window a real campaign is displayed , and only at this point your credits are used.
