All Buddha's doctrines can be summarized into two sections: the causal path and the resultant path. The meaning is that in the causal or sutra path it is emphasized that all sentient beings have Buddha nature, which is a cause of enlightenment. In the resultant path or Vajrayana it is introduced that Buddha is present in each sentient being. There it's emphasized that the nature of our three doors is the three kayas, and we meditate on it through the instructions and empowerments from authentic masters.
In the tantric path there is also the oral transmission and the terma transmission, or the long lineage and the near lineage. The meaning of the near lineage is that when Padmasambhava came to Tibet he gave many teachings, particularly many sadhanas such as the Great Sadhana of Eight Herukas, and many sadhanas of Guru Yoga. Through those teachings many Tibetans attained the Vidyadhara levels, for example the 25 disciples including King Trisong Detsen. Those teachings are very profound, and Padmasambhava said that after attaining realization through these teachings they have to be hidden for the benefit of future sentient beings. Padmasambhava also gave predictions, such as the time, place and person to reveal the teachings. Many treasure revealers have appeared exactly according to Padmasambhava's predictions. For example this Rigdzin Thugthig teaching was hidden by Lochen Vairochana and Khandro Yeshe Tsogyal with Padmasambhava into Samye Yamalong. Padmasambhava said that in the future Lochen Vairochana will reveal these teachings, spread them around the world, and this teaching will be very necessary for the beings of that time.
Lochen Vairochana appeared in this world exactly according to Padmasambhava's prediction in the form of Terdag Lingpa. At the age of 22 Terdag Lingpa had many visions of remembering his previous times with Padmasambhava, and many dakinis reminded him that he should go and reveal these teachings according to Padmasambhava's advice. Because of that he was compelled to reveal these teachings from their hiding places, where they were hidden exactly as predicted.
There were many hundred years between Padmasambhava and Terdag Lingpa, but Terdag Lingpa received these teachings as Lochen Vairochana directly from Padmasambhava. Therefore this lineage is very short. Because of the lineage being short, there are no broken samayas between Padmasambhava and the disciples. Therefore Terma treasures are more blessful and powerful than oral transmissions.
Rigdzin Thugthig is a sadhana of Guru Rinpoche. Guru Rinpoche is inseparable from Amitabha and Avalokiteshvara: the secret Dharmakaya is Amitabha, inner Sambhogakaya is Avalokiteshvara, and outer Nirmanakaya is Padmasambhava, who appeared in the lake of Danakosha for the great benefit of sentient beings in both India and Tibet. Padmasambhava's teachings are particularly significant for the beings of degenerate times. Rigdzin Thugthig is a Guru sadhana of Padmasambhava, and one of the main treasures of Terdag Lingpa. Since its founding by Terdag Lingpa until this day there is an unbroken lineage. It is the main Guru sadhana of the Mindrolling tradition, where it is practiced for great benefit of many sentient beings.