When speaking about mantra with kumbhak, it is important to have some understanding of both outer and inner tantra. Kumbhak means pot, and in the context of sadhana refers to the retaining of breath whereby the body becomes the pot. When working with a Vidya, the form of the Devata is the outer tantra, which means that the one doing the sadhana "assumes" that form. The mantra is the subtle vibration and expression which corresponds to the form. Mantra is also (or can be) the link/pin between the outer and inner tantra. This means that mantra can get one's vital elements to resonant in deep accord with the devata. One may put one's mind in full concentration on a Deity, but if it doesn't "hit home" (i.e. if one's vital experience is completely different), then there will be obstruction. The inner tantra refers to three main elements which when integrated form the complete expression. These elements are the vayus, nadis, and bindus. When the inner and outer tantra is in accord, then there is complete integrity of experience. There are several ways to bring about this integrity of experience, and each need not be independent of the others. Deep sincerity is even a path of integration, as one in this state will be very open, this openness in terms of inner tantra, loosens the patterns/holds of the vital elements which allows them to seamlessly integrate. Just the mantra japa (with outer tantra) may result in inner tantric accord, but one would really needs to have openness and then put the entirety of their Being in line with the mantra/devata. The more forceful way of developing this integration is performing the mantra with kumbhak. Before even starting practice with kumbhak, one should have a good amount of experience in the vidya and complete at least one purashcharan. Mantra when combined with kumbhak gets refined in unique ways, and the vital elements get refined as well. With the pot breath, the prana is pushed down, while the apana comes up, both meeting/mingling in the navel area. Ultimately, the energy should enter the central nadi, but traditions vary on particular processes and kriyas. The perfection of kumbhak leads to a deeper understanding and interaction with the bindu principle which is the very essence of Being. Bindu is also the vital principle in conception and forming new life. Bindu is also great bliss when experienced, and is the gateway to one's enlightened essence and compassion (as essential/vital quality rather than mere sentiment). While understanding the gross aspects of biology and conception (via seed and egg), bindu is the subtle principle which sparks consciousness and life. The bindu principle is a whole topic in itself, and is at the heart of several sadhanas which are widely misunderstood (most notably consort sadhanas), but suffice it to say that the mastery of kumbhak is the quickest path in greater understanding of bindu. One round of mantra on one kumbhak is the beginning level, so one practices to get to this level. Five rounds on one kumbhak is intermediate level, while ten plus shows some mastery. While the length of the mantra can differ (and to some extent determine one's immediate capacity), ultimately it is not much of a factor once a deeper understanding is gained. Proceeding in this more forceful way can be dangerous, so it is advised that one is guided by someone with real experience. Moreover, when conducting sadhanas which quickly hasten result, one should understand that everything is hastened. Therefore, latent tendencies, emotions, and even sicknesses may arise. It is very important that once one starts certain sadhanas that they are seen completely through. Much of the so-called "testing" that a sadhak may endure before receiving certain works, is about determining one's persistence, as persistence is very important for many practices.