这段代码检测浏览器Cookie是否打开，效果很好，可愣是没看懂

下面一段代码是检测浏览器Cookie是否打开，测试了IE6，IE7，Firefox3，safari, Opera9.5，测试结果都很正确，可就是没看懂？

function getCookieStatus() {
    var status ="<font color=\"red\">Cookie关闭</font>";
    var cookieStr = "wb_check=kcehc_bw";
    document.cookie = cookieStr;
    
    if (document.cookie.indexOf(cookieStr) > -1) {
        status = "Cookie打开";
        var date = new Date();
        date.setTime(date.getTime() - 1000);
        document.cookie = cookieStr + "; expires=" + date.toGMTString();
    }
    return status;
}