Quick question about the term "Pushdown Images"

I keep hearing people refer to using push down images to get their adsense ads to convert better. Can anyone please explain what the term means and the practical application of the term.

i.e. Are the ads above or below the image?

Good question. Shame there is no answer. Good question as in SEO terms this page showed up on Google No 1 slot for I too was searching for the answer...