While checking my email on my coporate GMail account today, I noticed that when I clicked on a button in GMail (eg, Inbox, or on a message), it starts to open when you press the button down, rather than when you release it to make the full click. This is not the typical behavior of a site (try it on any other hyperlink).
After pondering for a while, I realized that this must be a performance thing. It must take 50-100ms for one to complete a click. However, pinging google.com from here (home, on a wireless connection) takes about 70ms. That head start by capturing onmousedown must be enough to hide most of the delay in fetching stuff.
With a bit of further investigation, I found that Google Search uses the same thing. If you look at the html for a typical query, you will see onmousedown="return rwt(.... rwt appears to be a function that sends a request to google.com/url with a query string that tells what result was clicked. This would make sense in collecting stuff about
are the best results really on the top position. It's genius that they slip this tracking in the onmousedown so that it does not affect user performance.