The cycle of violence in abusive relationships typically follows the pattern below. But the cycle image is a bit misleading, for as time passes, the intervals between events shortens. So a more accurate image is a spiral as the circle gets tighter and tighter.*
The way this violence works is through repeated acts of power and control, which are reinforced by the threat or acting out of physically violence. Most people think of domestic/dating violence as occasional acts of physical violence. But in reality, it is repeated, daily acts of controlling the victim that grind away at her sense of reality and self-determination. It is the fear of physical violence that keeps her in line and in the relationship, and the honeymoon phase ("Phase 3") that traps her into denial that he could change.
This is why maintaining contact with victims of abuse is so important--her isolation makes it impossible for her to receive those "reality checks" that help her understand not only that he will not change (why should he; he doesn't need to as long as she is there), but that she is capable of getting free, with her children, if she has children.
* Cycle of violence artwork courtesy of The Women's Place, UVa Health Sciences Center.