As we know from the history Mahasiddha Matsyendranath was a Guru of Gorakshanath. Also Matsyendranath was a really famous and great Yogi and especially famous as a Tantric practitioner. He founded one of the big tantric sampradayas - the Kula Sampradaya - and influenced on many other doctrines. In India he is called the Kaula-Avatar - the manifestation of God who supports the way of Tantra in this Yuga (i.e. Kali-yuga now). In Nepal he was known as Minanath (Minapa) and Avalokiteshwara, and he was included in the lists of 84 Mahasiddhas of Nathas and Buddists.
So it seems understandable that Dada Guru Matsyendranathji added so much tantric points to the Nath Yogic Tradition at the time of its formation. And that's why there is a union of Shunyata (emptiness) and Purnata (fullness) as a high perfection of God in the Nath doctrine.
Let's consider the main tantric points in Nath Sampradaya.