Yoyo() to repeat a tween doesn't seem to work

1

Why the difference in tween and why doesn’t
circle_mc.onEnterFrame = pulse;
work?

vs

circle_mc.onEnterFrame = pulse();


import mx.transitions.Tween;
import mx.transitions.easing.*;

//circle_mc.onEnterFrame = pulse(); //uncomment comment to test
circle_mc.onEnterFrame = pulse;//uncomment comment to test
function pulse() {
	var yTween:Tween;
	var xTween:Tween;
	yTween = new Tween(this, "_yscale", Regular.easeOut, this._yscale, 200, 5);
	//xTween = new Tween(this, "_xscale", Strong.easeOut, this._xscale, 200, 5);
	yTween.onMotionFinished = function() {
    this.yoyo();
	}
}

with circle_mc.onRollOver = pulse; it does work
Anyone??