Well, there are actually three forces that dilate the cervix and force the fetus down the birth canal. So far, all of this has been involuntary. Once the kit is in the birth canal, the jill is in second stage and will feel a strong urge to bear down. Inexperienced jills interpret this as a bowel movement, which is why many a kit has been born in the litter box. Cephalic presentation is by far the most common.
Presentation is the part of the fetus presenting for birth.
There is also a face presentation in which the head is sharply flexed backwards. There is also a mentum presentation, in which the chin presents. Stay with me, we are not done yet. You can also get a transverse lie, which fortunately is not too common, that I know of, in ferrets. Transverse lie is the kit presenting sideways.