“The next four Sūtra refer to Hatha Yoga practices.

There is a Cakra in the body called the Nābhi Cakra also known as the Mañipūra Cakra.
By the practice of Saṃyama there arises out of Saṃprajñāta Samādhi and the Saṃyamā, Jñānam or understanding of the body and its arrangement.

The body here can be either gross or subtle and the understanding could be of nerves, vessels, and/or Nāḍī.
As a Hatha Yoga practice there are Āsana that are specifically stimulating to that area.
Such as Mastsyendrāsana, Baddha Padmāsana, Kandāsana, etc…

Apart from Āsanas there are other means to draw the attention to this area, in particular the practice of Uḍḍīyana Bandha.

The practice of Recaka Prāṇāyama and Bāhya Khumbaka also focusses there.
So the order would be:
Recaka Prāṇāyama + Bāhya Khumbaka + Uḍḍīyana Bandha + Mūla Bandha.
All bring the Bhāvana to the Nābhi Cakra.

Mūla Bandha is introduced here, as Uḍḍīyana is not complete and cannot be perfected without adding the practice of Mūla Bandha.
The practice of Mūla Bandha is aided by Mahā Bandha, sitting on the heels of the other foot in Ardha Padmāsana. The idea here is that sitting on the floor spreads out the buttocks, whereas sitting on one heel helps the Mūla Bandha and stimulates the Muladhara Cakra.
Then using Prāṇāyama and the Bandhas sit and perform Mantra.

Also for aiding Mūla Bandha, Aśvinī Mudrā can be practised 30-60 times in a posture.
This helps to improve the stamina for Mūla Bandha.

So for Mūlādhāra Cakra, Mūla Bandha.
For Svādiṣṭhāna Cakra and Maṇipūra Cakra, Uḍḍīyana Bandha.

Also in inverted postures, gravity helps the practice of Bandha and allows the breath to be smoother in effect.

Nābhi Cakra is called the centre of the body and postures like Mayūrāsana work around it.

So in this Sūtra, Manipura Cakra has been discussed and now Patañjali moves to Saṃyama on the Viśuddhi Cakra.

In the commentary Vyāsa relates the Jñānam to the three humours: wind, bile and phlegm and relates these to the disturbance of the qualities of Sattva, Rajas and Tamas.

Also understood are the seven corporeal elements, each more subtle than the previous one: Skin, Blood, Flesh, Sinew, Bone, Marrow and Semen.

Krishnamacharya diagnoses by the navel.
Sometimes checking the alignment with and distance from other parts of the body.
Sometimes by manipulation of the area around the navel, physically lifting and dropping the skin of the navel.”