ActionScript 3.0 :: Tween Class - Tween Back To The Original Position?

I am trying to understand the thinking behind motion tweening using AS3 (rather than doing it on the timeline).I am trying to create a simple platform game, and while this is simply a learning project, I have an movieclip instance on the stage, and using basic x+5 logic I can get the instance to move left and right on the stage accordingly.Trying to make the instance 'jump' is a bit trickier. This is the code I am using:

ActionScript Code:
import fl.transitions.Tween;
import fl.transitions.easing.*;
import fl.transitions.TweenEvent;[code].....

I thought this code would move the instance from it's current y-position and tween it to it's highest jump point. From here I planned to use a MOTION.FINISH event to tell it to tween back to it's original position.Something strage happens though, rather than tween from the current y position and move up, the instance instantly transforms it's position to y+100 and then tween back to the original position. So it kind of turns out that it is the second part of the jumping action that I want to create.