Extracted from the website of Buddhist Vajrayana Charity Funds Association and translated (方便道與解脫道) into English:

The Path of Skillful Means and The Path of Liberation
(An extract from Professor Zhenji Zhang’s writings)

The Path of Skillful Means (Sanskrit: upaya marga)
(1) Prerequisites
  Include the fundamental training of renunciation (the mind for leaving the passions of life) and Bodhi Mind (Sanskrit: Bodhicitta). It also consists of the overcoming of hindrance from harmful behaviour as well as the accumulation of necessities. In short, it is the cultivation of all kinds of preliminary practices that seek for blessings like the Four Initiation Practice, Guru Corresponding Dharma etc. which are the prerequisite practices before the cultivation of proper practices.
(2) Yidam Yoga
  It is the practice of visualising oneself as being the Yidam Buddha. Its functions are as follow:
  1. Visualising oneself as the Yidam Buddha, the universe becomes the Mandala Pure Land. Hence, it is a kind of training of the stage of insight that all are living Buddhas of the same spiritual nature.
  2. Before contemplating oneself into one’s Yidam, one first visualises everything that manifests be attributed to the great void (Sanskrit: the Mahayana Parinirvana, 大空); at the end of the meditation, one’s Yidam will be fused into the great void again. Therefore, it is a type of practice that trains the Visualization of Emptiness (Sanskrit: Sunyata, 空性).
  3. Chanting the mantra as well as visualising the seed-syllable, the Yidam and the mantra-wheel etc., one can obtain blessings and also the effectiveness of practising meditation.
  4. Visualising the immaculate chakras inside one’s body is the Yoga of cultivating the Correspondence of Sambhogakaya Buddha (報身佛). It is, simultaneously, setting the foundation for the Yoga of Identicalness of Mind and Prana (心氣).
  5. Because of the practice of Yidam Visualisation, Mandala Visualisation as well as Yoga of (light) collecting and emitting, one will thus be fearless in mind when the Bardo (中陰) state appears. On the contrary, one may catch the opportunity to attain liberation (Sanskrit: moksa) as well as realization instead.
    (1) & (2) above are regarded as the practice of the
Generation Stage (Sanskrit: Utpattikrama).
  6. When the practice of the Generation Stage is of pretty good foundation, one will further one’s practice to the Completion Stage (Sanskrit: Shavannakrama) which involves the advanced practice on Mind-Prana as well as the cultivation of Dream Yoga, the Light Yoga and the Practice of Illusory Body etc.
  7. As the practice of the Completion Stage has reached a certain level, one’s mind-prana can then be freed and it is so autonomous that prana is able to be guided into one’s Middle Vein (Sanskrit: Pingala Nadi, 中脈) to open the chakras throughout one’s body. At this stage, one’s mind will have achieved Emptiness whereas one’s prana will have become unobstructed (anavrti) miraculous power for the purpose of benefitting all beings.
  8. Since one’s mind-prana is completely free, one can thus be liberated from life and death as well as bardo so that one is, forever detached from reincarnation; on the other hand, one is able to liberate other beings among the Dharma Realm (Sanskrit: Dhammadhatu) depending on destiny with the acquired miraculous power and transformation.
  9. The supreme perfect condition for the attainment of Mind-Prana is the achievement of Buddhahood in which the three bodies of a Buddha, namely the Dharmakaya (法身), the Sambhogakaya (報身) and the Nirmanakaya (化身), are all satisfied.
The Path of Liberation (Sanskrit: vimukti marga)
The Path of Liberation, which is also called Path of Formlessness, is a comparatively simple and direct way of practice. This path enables one to go straight into one’s mind and recognise at once the Luminosity and Emptiness of Mind (明空心體) in which it is then being stabilized, expanded as well as intensified after that. The manifestation of the perfect completion of the Luminosity and Emptiness of Mind is exactly the attainment of a Dharmakaya Buddha. Hence, the key of this path is Guru’s instructions on the essence of mind at the very beginning. Upon knowing and receiving the essence of mind, the practitioner can immediately practise the mental-ground Dharma-gate (心地法門), which has neither form nor reliance. In Tantric Buddhism, it is known as Yoga of Mahamudra. The essentials for the practice of Mahamudra are relaxation, intentionlessness as well as brightness and luminosity. Among them, instinctive relaxation is of particular importance because all vexations and attachments are both a kind of ‘tense indication (laksana)’. Reincarnation (Samsara) is caused by ‘tenseness’ (attachment) as well. Comparatively, all realms of liberation (解脫境界) are ‘relaxed’ (detached) indications. Hence, ‘relaxation’ or ‘from tenseness to relaxation’ is a way to start with and solely ‘great tension as well as big relaxation’ can enable one to enter the gate of liberation (解脫之門).
There are many levels and stages in between relaxation and tenseness. In some of the stages, the achievement of relaxation also requires the effort of tenseness to make it workable. The practice of Mahamudra stresses itself on the instantaneous and nothing to practise (任運無修). Nevertheless, a pro forma ‘cultivation’ is still unavoidable and this is the practice of non-practice/practise without practising (無修之修). The ‘relaxation’ in Mahamudra is ‘letting go of everything. As one puts everything down, then the light of one’s Dharma body will naturally be revealed. Therefore, Mahamudra is also known as ‘The Dharma of Manifestation of Innate Pimordial Wisdom 開顯本覺俱生智之法’. Since the doctrine, realm and the realization of the attainment (Sanskrit: phalasacchikiriya, 證果) in Mahamudra are very similar to those of Chinese Chan (中國的禪宗), it can also be named as ‘Tibetan Zen’ (西藏禪).
‘Relaxation’ seems easy but actually, if neither an oral transmission (including the secret behind) nor instruction is accessible; it will be extremely difficult for one to approach. Even though one manages to ‘relax’, one’s accumulation of habitual tendencies (Sanskrit: vasana 習氣 means a past impression in the mind that influences behaviour) without a beginning is so critical that one needs to make exceedingly great efforts as well as having substantial cause-condition before one can achieve the practice of sustainable and in-depth ‘relaxation’.
The difference between the path of skillful and the path of liberation means appears only in their forms at beginning levels. In the two advanced stages, the two paths merge into a single one. After having pretty good achievements in the cultivation of Mahamudra, a great many practitioners turn around to practise the path of skillful means again.
Milarepa was a great achiever (Mahasiddha), who cultivated both paths concurrently.