Says Lama Zopa Rinpoche:
Trehor Kyörpon Rinpoche, a very high lama from Drepung, completed not just the first level but also the three higher levels of tantra. Highest Yoga Tantra is divided into generation and completion stages. The completion stage has the following levels: isolation of body, isolation of speech, isolation of mind, clear light, illusory body and unification of clear light and illusory body. After becoming a lharampa geshe and completing his monastic studies, he spent time living in the practice and meditating on what he had learned in order to make himself familiar with the practices and thus transform his mind into the sūtric and tantric path to enlightenment. Leaving the monastery with an assistant who was also a very great practitioner, he went to a very high mountain near Lhasa, whose top is always shrouded by clouds, to look for a meditation cave. He carried his yellow monks’ robes and one text—Lama Tsongkhapa’s Lamrim Chenmo. On the ascent a falling stone rolled by him and, following its path for a while, he came to a cave in which there was a skeleton, sitting in the meditation posture.
Rinpoche sat down and offered it a maṇḍala, and as he was doing so the skeleton collapsed. So Rinpoche decided to live in this cave to actualize the path by meditating on the lamrim. Gradually his disciples also came to meditate and gain experience and lamrim realizations. They were not in the same cave but scattered around— one here, one there—over quite a distance.
After he fled Tibet following the Chinese occupation, he lived in Dalhousie, which is not far from Dharamsala. Here he gathered a group of incarnate lamas and geshes who had studied the five scriptures that form the basis of the monastic curriculum—the Pramāṇavarttika, by Dharmakīrti; the Abhisamayālaṃkāra, by Maitreya; the Vinayasūtra, by Guṇaprabha, which is a commentary to the vinaya of Buddha Śākyamuni; the Abhidharmakośa, by Vasubhandu; and the Madhyamakāvatāra, by Candrakīrti, which has extensive teachings on emptiness, the two truths and also the path. Among those who requested him to guide them, Trehor Kyörpon Rinpoche selected only those who could really practice and gain realizations—ascetics able to live in pure Dharma, including the most learned lharampas, abbots and incarnate lamas from the three monasteries who had studied in Tibet—and to this select group, he gave extensive lamrim teachings. He passed away in Dalhousie in the mid-70s.
Anyway, to cut a long story short, he said,
“A person of high intelligence who has collected vast merit and has a great many imprints of the teachings on emptiness collected on his or her mental continuum will have no difficulty realizing emptiness. The one who will have difficulty is the one who has very little merit and few imprints.”
We can see how, even in this world, many human beings are born and die with totally empty lives; lives without meaning. In our own case, even if we are unable to practice in our daily lives, at least by reading about emptiness and listening to teachings on it, we leave positive imprints on our mental continuum and thus make preparation to realize emptiness in the near future. We will definitely find it much easier to understand the Buddha’s emptiness teachings as well as those commentaries by ancient highly attained pandits, yogis and scholars such as Lama Tsongkhapa. Emptiness realizations will also come more quickly. Then, through developing that wisdom we will be able to cease the gross and subtle defilements that are the mistakes of the mind. We will be able to achieve the sorrowless state that is the cessation of suffering together with its causes that we carry on our mental continuum. Additionally, we will be able to achieve full enlightenment, which means we attain the great liberation from even the subtlest mistakes of our mind.
 Tantric practice, according to the Gelug, can be divided into four levels: Kriyā (Action), Caryā (Performance), Yoga and Anuttarayoga (Highest Yoga) tantras. The latter is subdivided into generation and completion stages.
 These refer to the six levels of completion stage practice. As Ngawang Palden explains, the definition of the stage of completion is “a yoga in the continuum of a learner that arises from having caused the winds to enter, abide, and dissolve into the central channel by the power of meditation.” He elaborates, “In the stage of completion, one is actually transformed into the deity one has merely imagined on the stage of generation. Within the vivid visualization of oneself as a deity and one’s environment as divine, one practices penetrative focusing on important points of the body, causing the currents of energy (winds) that course through the body to enter and dissolve in the central channel and then into the indestructible drop in the center of the heart. Using the very subtle and powerful mind of “clear light” that dawns as a result of the total dissolution of winds in the indestructible drop, one realizes emptiness. Rising up in the form of the deity in an illusory body—a body made of wind—and meditating on emptiness with a mind of clear light, one swiftly amasses the collections of wisdom and method, overcomes all of the obstructions to liberation and omniscience, and achieves the final rank of Buddhahood.” See Daniel Cozort, Highest Yoga Tantra (Ithaca: Snow Lion Publications, 1986), 65–66. The implication is that Trehor Kyörpon Rinpoche had actualized the entire path. For a comprehensive account of completion stage practice, see Kirti Tsenshap Rinpoché, Principles of Buddhist Tantra (Boston: Wisdom, 2011).
 See Tsongkhapa, The Great Treatise on the Stages of the Path to Enlightenment, Volumes One–Three (Ithaca: Snow Lion, 2000, 2002, 2004).