“To start with, Nimitta is usually an external agency, but this
external agency must itself also be linked to some Nimitta.
In early life for example, our parents, friends or teachers can
be a Nimitta.
As to Yoga, the best external Nimitta is a teacher.
Here, the teacher’s role is to help guide you towards your internal
Nimitta, in that the teacher will get to know you and support you
with the appropriate means for your journey from outer to inner.
From this process, a mirror for Nimitta can be through any number
of things, for example Dharma or even Duḥkha can offer guidance.”
– Paul Harvey on Yoga Sūtra Chapter Four verse 3

“Janu Śīrṣāsana, known as the head to knee Āsana,
combines the qualities of a seated forward bend with
those of an asymmetrical pose, in that it works first,
on one side of the body more, then on the other side.
In this Āsana, the physical focus is on the lengthening
of the muscles of the back, spine and the extended leg,
and the rotational effects on the joints in the folded leg.
Plus, with its seated forward bend Bhāvana, there is
a natural support favouring the exhalation, offering
opportunities to both lengthen and deepen the breath.
This support, along with its  internalising and closing
qualities, offers possibilities for the student to focus on
the quality of the attention within the form of the body.
Thus, using Janu Śīrṣāsana with its seated forward bend
Bhāvana, and when linked with a mindful breath, can help
the student to experience the deeper function of the Āsana,
even if there are limitations with regard to its Śikṣaṇa form.”
– 108 Postural Practice Pointers