下記のコードは、文字列がマウスポインタに追従するものですが、これをトグルボタンを使ってマウスポインタと追従する文字列を切り離したり(off)、再び追従させたり(on)したいのですが、文を組み立てられません。スクリプトの一番下にある
if (window.addEventListener){}
else if (window.attachEvent){};
に鍵があるのは何となく分かるのですが、どのように書き加えればいいでしょうか。

<!DOCTYPE html>
<html lang="ja">
<head>
<meta http-equiv="Content-type" content="text/html; charset=Shift_JIS">
<meta http-equiv="Content-Script-Type" content="text/javascript">
<meta http-equiv="Content-Style-Type" content="text/css">
<title>文字列の追従(ON⇔OFF)</title>


<style type="text/css">

#myText {
/* Optional - DO NOT SET FONT-SIZE HERE, SET IT IN THE SCRIPT */
font-style: italic;
font-weight: bold;
font-family: 'comic sans ms', verdana, arial;
color: #ff0000;

position: absolute;top: 0;left: 0;z-index: 3000;cursor: default;}
#myText div {position: relative;}
#myText div div {position: absolute;top: 0;left: 0;text-align: center;}

</style>

<script type="text/javascript">
<!--

;(function(){

// Your message here (QUOTED STRING)
var msg = "文字列の追従(ON⇔OFF)"; 
var size = 24;
var circleY = 0.75; 
var circleX = 2;
var letter_spacing = 5;
var diameter = 10;
var rotation = 0.4;
var speed = 0.3;


if (!window.addEventListener && !window.attachEvent || !document.createElement) return;

msg = msg.split('');
var n = msg.length - 1, a = Math.round(size * diameter * 0.208333), currStep = 20,
ymouse = a * circleY + 20, xmouse = a * circleX + 20, y = [], x = [], Y = [], X = [],
o = document.createElement('div'), oi = document.createElement('div'),
b = document.compatMode && document.compatMode != "BackCompat"? document.documentElement : document.body,

mouse = function(e){
 e = e || window.event;
 ymouse = !isNaN(e.pageY)? e.pageY : e.clientY; // y-position
 xmouse = !isNaN(e.pageX)? e.pageX : e.clientX; // x-position
},

makecircle = function(){ // rotation/positioning
 if(init.nopy){
  o.style.top = (b || document.body).scrollTop + 'px';
  o.style.left = (b || document.body).scrollLeft + 'px';
 };
 currStep -= rotation;
 for (var d, i = n; i > -1; --i){ // makes the circle
  d = document.getElementById('iemsg' + i).style;
  d.top = Math.round(y[i] + a * Math.sin((currStep + i) / letter_spacing) * circleY - 15) + 'px';
  d.left = Math.round(x[i] + a * Math.cos((currStep + i) / letter_spacing) * circleX) + 'px';
 };
},

drag = function(){ // makes the resistance
 y[0] = Y[0] += (ymouse - Y[0]) * speed;
 x[0] = X[0] += (xmouse - 20 - X[0]) * speed;
 for (var i = n; i > 0; --i){
  y[i] = Y[i] += (y[i-1] - Y[i]) * speed;
  x[i] = X[i] += (x[i-1] - X[i]) * speed;
 };
 makecircle();
},

init = function(){ // appends message divs, & sets initial values for positioning arrays
 if(!isNaN(window.pageYOffset)){
  ymouse += window.pageYOffset;
  xmouse += window.pageXOffset;
 } else init.nopy = true;
 for (var d, i = n; i > -1; --i){
  d = document.createElement('div'); d.id = 'iemsg' + i;
  d.style.height = d.style.width = a + 'px';
  d.appendChild(document.createTextNode(msg[i]));
  oi.appendChild(d); y[i] = x[i] = Y[i] = X[i] = 0;
 };
 o.appendChild(oi); document.body.appendChild(o);
 setInterval(drag, 25);
},

ascroll = function(){
 ymouse += window.pageYOffset;
 xmouse += window.pageXOffset;
 window.removeEventListener('scroll', ascroll, false);
};

o.id = 'myText'; o.style.fontSize = size + 'px';

if (window.addEventListener){
 window.addEventListener('load', init, false);
 document.addEventListener('mouseover', mouse, false);
 document.addEventListener('mousemove', mouse, false);
  if (/Apple/.test(navigator.vendor))
   window.addEventListener('scroll', ascroll, false);
}
else if (window.attachEvent){
 window.attachEvent('onload', init);
 document.attachEvent('onmousemove', mouse);
};

})();
// -->
</script>


</head>
<body bgcolor="#ccffff">

</body>
</html>