they\'re using CSS3 transitions.
<style type="text/css">
#s {
float: right;
-webkit-transition-duration: 400ms;
-webkit-transition-property: width, background;
-webkit-transition-timing-function: ease;
-moz-transition-duration: 400ms;
-moz-transition-property: width, background;
-moz-transition-timing-function: ease;
-o-transition-duration: 400ms;
-o-transition-property: width, background;
-o-transition-timing-function: ease;
width: 72px;
}
#s:focus {
background-color: #f9f9f9;
width: 196px;
}
</style>
<input type="text" class="field" name="s" id="s" placeholder="Search">