The various kalas are around Devi’s chest back yoni etc.. When Shiva and Devi are in union when HIS hands touch HER waist etc., a ray of light passes through them and the mantras are formed. For example the mantra GamPham is formed by the ray passing between Ga at the breast and Pha at her waist as shown by the arrowed line. So while resiting the mantra GamPham visualize a ray of light passing between these two letters.
