Siddhas are those who have perfected themselves in the field of enlightenment.

Verse 3.32

कूर्मनाड्यां स्थैर्यम् ॥३२ ् ॥

kūrma-nāḍyāṁ sthairyam ॥32॥

On the nerve called Kurma (comes) fixity of the body.

Verse 3.33

मूर्धज्योतिषि सिद्धदर्शनम् ॥३३॥

mūrdha-jyotiṣi siddha-darśanam ॥33॥

On the light emanating from the top of the head sight of the Siddhas.

By mastery over kurma nadi, the yogi not only keeps his physical body immobile like a tortoise, alligator or snake, but also has the power to hibernate mentally by completely immobilizing the functions of the body and intellect.

 Karma nadi corresponds to the epigastric region ( it is the upper central region of the abdomen).

Siddhas are those who have perfected themselves in the field of enlightenment.

Murdhajyoti represents the ajna cakra of the yoga texts.

A yogi can develop a balanced head , and from his visions of siddhas, yogis and Acharya's (great teachers) he may obtain guidance and inspiration to further his sadhana.

PATANJALI YOGA SUTRAS