A Pushdown creative is like an expanding ad, with one difference--it pushes down the content of a webpage when the creative expands, moving the site out of the way to display the ad.
Pushdown functionality isn't built into the Flash assets of the creative. There's a pushdown setting in DFA or DFP that's applied when the ad is trafficked, and you can preview this functionality using Display Options in the DoubleClick Studio Web UI.
Because pushdowns only work for creatives that expand down, the option for pushdown functionality is only available for creatives with the Expanding component set to an X,Y position of 0,0 in the Flash file.
Set your creative to push down page content
In the Preview tab, click Creative Properties and check the Pushdown checkbox.