哪位高手可以详细解释下js轮播器中的缓冲效果实现原理？

小弟看别人写的js轮播器，到了处理缓冲效果那里就不懂了，希望哪位大侠可以帮忙详细解释下，谢谢！<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gbk">
<title>测试</title>
<meta name="author" content="ximan">
<meta name="keywords" content="">
<meta name="description" content="">
<link type="text/css" rel="stylesheet" href="index.css">
</head>
<body>
<div class="content">
<ul id="num">
<li style="background: #f00;">
1
</li>
<li>
2
</li>
<li>
3
</li>
<li>
4
</li>
<li>
5
</li>
</ul>
<ul id="img_box">
<li>
<a href="#">
<img src="pailabi_shop1.jpg" alt="哈哈哈" />
</a>
</li>
<li>
<a href="#">
<img src="pailabi_shop2.jpg" alt="哈哈哈" />
</a>
</li>
<li>
<a href="#">
<img src="paila_shop3.jpg" alt="哈哈哈" />
</a>
</li>
<li>
<a href="#">
<img src="paila_shop4.jpg" alt="哈哈哈" />
</a>
</li>
<li>
<a href="#">
<img src="paila_shop5.jpg" alt="哈哈哈" />
</a>
</li>
</ul>
</div>
<script type="text/javascript">
window.onload=function (){
var num = document.getElementById("num");
var num_li = document.getElementById("num").getElementsByTagName("li");
var img_box = document.getElementById("img_box");
var img_box_li = document.getElementById("img_box").getElementsByTagName("li");
var bliw = img_box_li[0].offsetWidth;
var n = 0;
img_box.style.left = 0
var autoscroll = setInterval(auto,3000);
for (var i = 0;i < num_li.length;i++){
num_li[i].onmouseover = function(){
clearInterval(autoscroll);
for (var i = 0;i < num_li.length;i++){
num_li[i].style.background = "";
if(num_li[i]==this){
this.style.background = "#f00";
slide(i);
}
}
}
num_li[i].onmouseout = function(){
for (var i = 0;i < num_li.length;i++){
if(num_li[i]==this){
n = i;
autoscroll = setInterval(auto,3000)
}
}
}
}
/**function slide(i){
img_box.style.left = -bliw*i + "px";
}**/
function slide(i){//就是这个函数里面的东西看不太懂，特别是两个if
                if(scroll) clearTimeout(scroll);
                (function(){
                        var left=parseInt(img_box.style.left);        //ul当前left的数值
//alert(left);
                        var pos=i*img_box_li[0].offsetWidth;        //距离第i张图片的距离
//alert(pos);
                        if(-1*left<pos){
                                var speed=Math.ceil((pos+left)/15);
                                img_box.style.left=left-speed+"px";
                                scroll=setTimeout(arguments.callee,20);
                        }
                        if(-1*left>pos){
                                var speed=Math.floor((pos+left)/15);
                                img_box.style.left=left-speed+"px";
                                scroll=setTimeout(arguments.callee,20);
                        }
                })()
        }
function auto(){
n++;
if(n == img_box_li.length){
n =0;
}
for (var i =0;i < num_li.length;i++){
num_li[i].style.background = "";
}
num_li[n].style.background = "#f00";
slide(n);
}
}
</script>
</body>
</html>