The picture below shows a cervix that is dilating. The top left shows a cervix that is not dilated or effaced. The top right show early effacement (thinning out). See how the effacement is making the cervix shorter. The bottom left show the cervix completely effacted, but not fully dilated. The bottom right shows the cervix to be completely dilated and effaced. As you can see the cervix must thin out before it can "shrink up" and begin to dilate. 

During childbirth, the cervix is opened (dilated) and thinned out (effaced) by the contractions of the uterus. When you have a contraction, the cervix pulls up. This action is sort of like trying to put on a turtle neck sweater. Imagine putting your head through a turtle neck without gathering up the long neck. You will have a hard time putting it on. Now imagine putting on a turtle neck after gathering the neck up. This is much easier. This is what your cervix must do before opening up.