Look at the initial markup:
<a onmousedown="return rwt(this,'','','','3','AFQjCNF8xnW_qOvuZURbtcZUvB6zhKtRQw','35cXyZwuoZY8hBY1VfDr8Q','0CEAQFjAC','','',event)"
href="http://en.wikipedia.org/wiki/How_Do_You_Do_It%3F">
<em>How Do You Do It</em>? - Wikipedia, the free encyclopedia
</a>
Initially the href
attribute shows the "real" URL. But when you click on the link, the rwt
function changes the attribute value to
http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&ved=0CEAQFjAC&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FHow_Do_You_Do_It%253F&ei=8nSMUqXtLKfe4QSzwIDADQ&usg=AFQjCNF8xnW_qOvuZURbtcZUvB6zhKtRQw&sig2=35cXyZwuoZY8hBY1VfDr8Q&bvm=bv.56643336,d.bGE
To answer your question: they use the onmousedown
attribute to change the link's href
attribute when you click on it. Barmar points out why they do it.