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.
First of all we can name shabar mantras that are widespread among the nath-yogis. In short, it's a special type of mantras made by Matsyendranath for the yogis, sadhus, tapasvi and etc., which depends on the power of Shiva Adinath. And although the shabar mantras are not clearly Tantric mantras (neither Puranic nor Vedic) there is a special shabar tantra describing the different ways of their use. It is also included rituals with yantras.
The second point is the attention to Kundalini Shakti and to the whole system of kundalini-jagaran sadhana. I mean chakras, nadis, five pranas, the inner alchemy for the awakening of each chakra and getting the Amrita (the nectar of immortality). At the base of them all are the various tantric knowledges. The chakras originated from Five Faces of Shiva (some speak about Four or Six Faces) that are related with different tantric traditions (amnaya). And each chakra contains some Vidya (tantric knowledge), inside. So, when we open our chakras we start realize that knowledge. Here we should mention as well that Kundalini came to yoga from ancient tradition called Kubjika. Kundalini is the name of Devi Kubjika. There is one legend where according to the advice of Shiva Kubjika started traveling around the India, visiting Shakti-Pithas, to get the knowledge of each Pith (vidya of local Deity of the Pith). So in this way, in our body Kundalini Shakti wanders from chakra to chakra (from one petal to another) and joins with Shiva at the end, and during this process we obtain the knowledge of whole body and all vidyas.
Another point is that some mudras, bandhas and adharas are also related with main tantric Shakti-Pithas, namely Purnagiri, Uddiyana, Jalandhara, Kamarupa. Uddiyana Pith is connected with Uddiyana-bandha, with kanda. Kamarupa (Kamakhya) Pith is connected with muladhara and mula-bandha. Jalandhara Pith is located at the neck area or in the head and connected with jalandhara-bandha. And Purnagiri Pitha is located at the top of the head and connected with the practices of Raja-yoga, with unmani-mudra and shambhavi mudra.
Also besides of main initiations in Nath Sampradaya (that are choti and chira dikshas) there is a special upadeshi-diksha that is related with the worship of Shakti. There is a one main Shakti named Yoga Maya Bala Sundari who is a Maya itself in the concept of Nath Tradition. She controls all Shaktis of the Universe, all that is manifested by Para Shiva. She is represented in young and beautiful form ("bala sundari") that also symbolizes the brahmacharya and vairagya, so She is a yogini also. Natha-yogis worship Her for different things - Shakti can do anything in this world. But the main aim of that worship is the way to Moksha. Because Yoga Maya can reduce the bond with Maya/samsara and She can let you go to Moksha and Mukti.