Ordinarily, japa starts in ... vocal muttering. It depends entirely on the will and activity of the person who does the japa. After constant practice of japa for some years, an extraordinary thing happens. A time comes when the japa does not depend on the will and activity of the reciter any longer. It now goes on automatically inwardly without any effort on the part of the reciter. It becomes an ajapâjapa. When this proceeds for a long time, the prâna and apâna currents ... become equilibrated; the kundalini now awakens; the equilibrated current now flows upward in the susumna i.e. in the interior of the spinal column. This upward movement is known as ucarana.... As the kundalini rises, there is the experience of anâhata nâda - automatic, unstruck sound. The kundalini passing through the various cakras finally joins the Brahmarandhra, and then nada {i.e. sound} ceases; it is then converted into jyoti (light).
(p. xxxiv)
Japa freed of all ideas of ens {i.e. existence} and non-ens {i.e. non-existence} is the constant contemplation of Siva's nature.
(p. 134, commentary on verse 145, quoting Abhinavagupta in Tantraloka (I, 90) )