From RCR's article Back Limbo Forward
In the beginning the light at the tunnel's entrance offers some clarity. As he goes farther it becomes darker and darker, but since they are not yet completely without light, they cycle. As he proceeds farther into the tunnel, the light at the beginning is no longer visible and the light at the end is not yet visible. Things get worse. The alienator relationship pulls him down. In the beginning he hated and loved you at the same time, often changing his mind every few minutes/days/weeks. Now, in the darkness, he seems even more certain he hates you, more consistent. He is gone without Hope of returning. He doesn't want to return. In this place of Perpetual Darkness he knows he made the right decision.
The Dark period can be very long. As he approaches the end of the tunnel he will see the light. He may cycle again--hating and loving you. The Rollercoaster is wildest at the beginning and end when they are making decisions. When he sees the light at the end of the tunnel, the damage also becomes visible. It may be so great that he runs again. Or he sees the Love and Hope in his spouse and becomes afraid.