Twitter uses the following code to prevent clickjacking:

<script type="text/javascript">
if (window.top !== window.self) {
document.write = ""; // 1
window.top.location = window.self.location; //2
setTimeout(function(){document.body.innerHTML='';},1); // 3
window.self.onload=function(evt){document.body.innerHTML='';}; //4
}
</script>

This uses four separate methods to prevent clickjacking, and there's some that I don't fully understand.

Method 1 overwrites the 'document.write' method, but I'm not sure what this prevents
Method 2 is the basic framebusting technique.
Method 3 deletes the content of the page to prevent it being clicked. This is needed in case the framing page uses some sort of anti-framebusting technique (e.g http://coderrr.wordpress.com/2009/02/13/preventing-frame-busting-and-click-jacking-ui-redressing/)
Method 4 deletes the content of the page once it has loaded - but I'm not sure why this is needed as well as method 3.

Does anyone have any ideas on why methods 1 and 4 are required?