They don't zoom in, what is really happening is that when you click the Zoom text, the image inside the div is replaced by a larger version of that image. And overflow is set to hidden.
As you move your cursor, using some clever JavaScript and DOM handling, the image is simply moved relatively accordingly, creating the effect you are actually zooming in the picture.
I'll try to create up a simple example for you.
EDIT
Sorry took a while but here it is:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<title>Example</title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script type="text/javascript">
$(function() {
var fullWidth = 864; // Width in pixels of full-sized image
var fullHeight = 648; // Height in pixels of full-sized image
var thumbnailWidth = 389; // Width in pixels of thumbnail image
var thumbnailHeight = 292; // Height in pixels of thumbnail image
// Set size of div
$('#picture').css({
'width': thumbnailWidth+'px',
'height': thumbnailHeight+'px'
});
// Hide the full-sized picture
$('#full').hide();
// Toggle pictures on click
$('#picture').click(function() {
$('#thumbnail').toggle();
$('#full').toggle();
});
// Do some calculations
$('#picture').mousemove(function(e) {
var mouseX = e.pageX - $(this).attr('offsetLeft');
var mouseY = e.pageY - $(this).attr('offsetTop');
var posX = (Math.round((mouseX/thumbnailWidth)*100)/100) * (fullWidth-thumbnailWidth);
var posY = (Math.round((mouseY/thumbnailHeight)*100)/100) * (fullHeight-thumbnailHeight);
$('#full').css({
'left': '-' + posX + 'px',
'top': '-' + posY + 'px'
});
});
});
</script>
<style type="text/css">
#picture {
overflow: hidden;
position: relative;
border: 1px solid #000000;
}
#thumbnail {
cursor: pointer;
}
#full {
position: relative;
cursor: crosshair;
}
</style>
</head>
<body>
<div id="picture">
<img alt="" id="thumbnail" src="http://img202.imageshack.us/img202/8403/93629325.jpg" />
<img alt="" id="full" src="http://img111.imageshack.us/img111/4734/tbmecsekyellowflower.jpg" />
</div>
</body>
</html>
You will need to adjust the thumbnail and full-sized image width and height accordingly. But simply copy paste the above to see an example of an image hosted on imageshack.