Mahayoga, anuyoga, and atiyoga all contain the same basic elements of practice on the tantra path to enlightenment. They differ in terms of emphasis. The same distinction is true concerning the three divisions of anuttarayoga tantra: father, mother, and nondual tantra. The two division schemes, however, are not equivalent.
The practice of anuttarayoga tantra entails the generation stage (bskyed-rim) and the complete stage (rdzogs-rim, completion stage). On the generation stage, we generate ourselves as Buddha-figures merely with the power of our imaginations (visualization). On the complete stage, everything is complete for actually generating the immediate causes for the body and mind of a Buddha – not just in our imaginations.
On the complete stage, we cause the energy-winds (rlung, Skt. prana) to enter, abide, and dissolve in the central channel. This enables us to access the subtlest level of mental activity (clear light, ‘ od-gsal) and use it for the nonconceptual cognition of voidness – the immediate cause for the omniscient mind of a Buddha. We use the subtlest level of energy-wind, which supports clear light mental activity, to arise in the form of an illusory body (sgyu-lus) as the immediate cause for the network of form bodies (Skt. rupakaya) of a Buddha.
Within this scheme of anuttarayoga tantra:
•father tantra emphasizes illusory body practice,
•mother tantra emphasizes clear light practice,
•nondual tantra emphasizes the unified pair (zung-’jug) of illusory body and clear light.
If we use the same scheme as in anuttarayoga to analyze the Nyingma presentation, and divide the complete stage into two phases – before attaining actual clear light nonconceptual cognition of voidness, and after, when we attain the immediate cause for a rupakaya – then:
•mahayoga emphasizes the generation stage,
•anuyoga emphasizes the first phase of the complete stage and working with the energy-winds, channels, and chakras,
•atiyoga (dzogchen) emphasizes the second phase of the complete stage, at which we actualize the immediate causes for the enlightening mind and form bodies of a Buddha.
Read more at :
http://www.berzinarchives.com/web/en/archives/advanced/dzogchen/basic_points/major_facets_dzogchen.html