Preventing/Fixing holes at the roots where the dread has split in two. - Dreadlocks FAQ's
Occasionally a dread will begin dreading in two parts. This creates an upside down Y shape. Sometimes this happens on it's own and sometimes it's a direct result of clockwise rubbing dreads with longer roots and letting the roots spread apart while you rub.
Unless you're trying to split a large dread into two dreads you're going to want to avoid this, here's how:
Since it can only happen when a dread knots and locks while it's roots are separated in two (or more) parts you have to make sure that knotting happens when the roots are all held close together. If you are clockwise rubbing this is pretty easy to do just by giving the dread one clockwise twist before you start. You'll also use your fingers to keep all the hair concentrated in the center as much as possible as you rub. Most of this is technique that you will develop as you work with it but by knowing what you are going for it becomes way easier. =]
If you have a dread that continues to try to split apart on you, which sometimes happens with dreads that are growing on cowlicks or other anomalies in your hair growth pattern, you can put a rubber band on the roots (snug, never tight). This will keep everything closer together and prevent the splitting. For more info on using bands at the roots see this article.