What is the method for "Coupling of the Serenity and Insight Powers" referred to in the Path of Discrimination? Visuddhimagga (English translation by Bhikkhu Nanamoli) XXII.46 quotes from the Path of Discrimination (Patisambidhamagga) as follows:-


  "Coupling of the Powers'. At the time of developing the eight mundane attainments the serenity power is in excess, while at the time of developing the contemplation of impermanence, etc., the insight power is in excess. But at the Noble Path moment they occur coupled together in the sense that neither one exceeds the other"


The quote concludes by stating


  "He develops serenity and insight coupled together in the sense of emergence (Patisambidhamagga ii,98)".


My understanding of the above is that the yogi needs to develop the method to ensure that coupling occurs at the Noble Path Moment. 

My question is anyone able to throw some light on this coupling from the scriptures or from some other source or from personal experience?

Discovered later that a possible answer to this question may be found in "Foundations of Tibetan Mysticism" pdf by Lama Govinda' p. 175. where reference is made to three zones viz. Earth (materiality), Universe and Human. The Human Zone is the plane of ''individual realization in which the forces of the earth and the universe become conscious in the human soul as an ever present and deeply felt reality'. If Earth and Universe could be related to Samatha and Vipassana we may be homing in on a solution to a problem that has bedeviled many a good yogi-that of coupling (yuganaddha) both these entities to realize Nibbana. Would appreciate a feedback, especially from those who are familiar with Vajrayana and ''Buddhist Kundalini'' energies.
"He develops serenity and insight coupled together" is originally found in the Pali sutta MN 149, as follows:


  When the noble eightfold path is developed, the following are fully
  developed: the four kinds of mindfulness meditation, the four right
  efforts, the four bases of... power, the five faculties, the five
  powers and the seven awakening factors.
  
  Tassa evaṃ imaṃ ariyaṃ aṭṭhaṅgikaṃ maggaṃ bhāvayato cattāropi
  satipaṭṭhānā bhāvanāpāripūriṃ gacchanti, cattāropi sammappadhānā
  bhāvanāpāripūriṃ gacchanti, cattāropi iddhipādā bhāvanāpāripūriṃ
  gacchanti, pañcapi indriyāni bhāvanāpāripūriṃ gacchanti, pañcapi
  balāni bhāvanāpāripūriṃ gacchanti, sattapi bojjhaṅgā bhāvanāpāripūriṃ
  gacchanti.
  
  And these two qualities proceed in conjunction: Tassime dve dhammā
  yuganandhā vattanti—
  
  serenity and discernment. samatho ca vipassanā ca.
  
  https://suttacentral.net/mn149/en/sujato


A reason why samatha can be unbalanced and not have vipassana is because the practitioner is actually not practising the Noble Path but, instead, is practising the Hindu style yogic manipulations that are actually taught in the Visuddimagga. 

In the suttas (MN 118 and SN 48.10), the Buddha taught meditative factors are mature when they have the quality of "letting go" ("vossagga"). This is the practise of the Noble Path, namely, the "letting go" or "abandoning of craving". 

If you try to watch the breathing, for example, or watch kasina coloured discs, and similar non-sense taught in the Visuddhimagga &amp; Abhidhamma, this is craving; this is not practising the Path. This is why most 'Buddhists' make little progress with meditation. They try to cling to a meditation object, like a baby clings to its mother. They are too scared &amp;/or too attached to wrong teachings to 'let go'. 



As for vipassana (insight) being overbalanced, this sounds like illogical (even though there is one or two suttas that teach similar non-sense). Since the suttas (such as SN 22.59; MN 118; etc) correctly explain vipassana results in dispassion; then obviously a dispassionate mind will be calm or have samatha.