I'm stumped. On (release) works but not onRelease

How is it possible that this works.

on (release) {
    _root.thumbs_mc.contentMain.damebilde1._x += 5;
    }

But this doesn’t work?

_root.thumbs_mc.contentMain.damebilde1.onRelease = function () {
    trace ("onRelease called");
};

damebilde1 is a movieclip, by the way.