In the last few weeks I stumbled over a few paid4 sites which pay their users for bounce rate manipulation. They usually guide the users through a few steps like:
1. visit google and search for the keyword "X"
2. visit one search result which ranks above "paid4partnersite.com" and hit your back button immediately
3. now click on the search result of "paid4partnersite.com" and stay for about 90 seconds, after that visit some of the subpages
As soon as the user did those steps he'll get credited for his work.
Does that really improve the rankings of a site? This would be ridiculous and even easier as buying some links.