The interstitial or countdown embed method places an intermediate page between the initial page load, and the actual viewing of the content. Here is an example of an UpRival interstitial ad:
There are options to set how long the ad will countdown for (in seconds), how many seconds the page will be shown before the interstitial ad appears (in seconds), and after how many seconds the ad will be skippable.
To create an interstitial ad, simply create your placement and ad as normal, and then select "Interstitial (countdown)" from the drop-down menu labeled “Embed Method Types”. This menu is found by clicking the “Get HTML Tag” button.