Scroll Button With Ease Effect In As3

I'm italian, so sorry for my english.

I'm working on my new website, and in a section of it I want a scroll button with ease effect.
You can see an example WITHOUT ease effect on the following link:
http://www.chaosmonger.com/chaos08_video.html

Made with this code:

Code:

---

btnUp.addEventListener(MouseEvent.MOUSE_OVER, BUTUP);
btnUp.addEventListener(MouseEvent.MOUSE_OUT, stopBUTUP);
btnDown.addEventListener(MouseEvent.MOUSE_OVER, BUTDOWN);
btnDown.addEventListener(MouseEvent.MOUSE_OUT, stopBUTDOWN);
btnUp.addEventListener(Event.ENTER_FRAME, BUTUP);
btnDown.addEventListener(Event.ENTER_FRAME, BUTDOWN);

contentMain.addEventListener(Event.ENTER_FRAME, skrolldown);
contentMain.addEventListener(Event.ENTER_FRAME, skrollup);

var scrolldown:Boolean=false;
var scrollup:Boolean=false;

import flash.events.Event;

    var scrollHeight:Number = 460;
    var contentHeight:Number = contentMain.height;
    var maskHeight:Number = maskedView.height;
    var initContentPos:Number = contentMain.y;
    var finalContentPos:Number = maskHeight-contentHeight+initContentPos;
    var speed:Number = 5;

   
    function BUTUP(event:MouseEvent):void
    {
        scrollup=true;
    }
       
    function stopBUTUP(event:MouseEvent):void
    {
        scrollup=false;
    }
   
    function skrollup (evt:Event):void
        {
            if(scrollup)
            {
                if (contentMain.y+speed<maskedView.y) {
                    btnDown.visible = true;
                    contentMain.y += speed;}
                else {
                btnUp.visible = false;
                contentMain.y = maskedView.y;}
            }
        }

    function BUTDOWN(event:MouseEvent):void
    {
        scrolldown=true;
    }
   
    function stopBUTDOWN(event:MouseEvent):void
    {
        scrolldown=false;
    }
       
    function skrolldown (evt:Event):void
        {
            if(scrolldown)
            {
                if (contentMain.y-speed>finalContentPos) {
                    btnUp.visible = true;
                    contentMain.y -= speed;}
                else {
                    btnDown.visible = false;
                    contentMain.y = finalContentPos;}
            }
        }
           
   
   
if (contentMain.y==0) {
        btnUp.visible = false;
    }

---

I have two buttons (btnUp & btnDown), a movieclip that contains all the images (contentMain) and a movieclip as a mask (maskedView).

I need an ease effect when I rollover/rollout or click the mouse, like this:
http://www.animallogic.com/#Our%20Work,Commercials

Can anybody suggest me a code to give an ease effect to buttons, without upset my original code?

THANKS A LOT!