Those who have practiced Vinyasa styled flow-yoga sequences in very traditional systems of yoga such as the Ashtanga Vinyasa system, would often have heard their teachers say, "Don't lose the thread!", when their students pause asanas to either chit chat amongst themselves or to go into a reverie. The use of the bandhas and the Ujjayi breath harness the prana to flow in a channeled fashion in the body. This creates a very precious state for the mind in which the mind is without distractions and worries and is at ease without any intervention or drugs. By channeling the energy in the body, normally effortful tasks like balancing on one's arms are achieved with much less effort.

As seen in this slide of "The subtle aspects of Hatha Yoga", the Ujjayi breath and the bandhas act like a blacksmith's below to stoke the fire in the body and make the prana flow effectively and in a channeled fashion. Asana practice, according to the Hatha Yogic texts, is channeling of the prana to awaken the normally dormant Goddess Kundalini to induce her to flow through the central Nadi called the Sushumna nadi.

References

1) Flexibility vs Malleability in Asana Practice

2) "The subtle aspects of Hatha Yoga" - workshop recording and slides

3) "Finding Moola Bandha"- workshop recording and slides

4) Know the rules of yoga well to break them correctly

5) Breath, Bandha and Agni(Fire) :The core of asanas in Hatha Yoga

6) Prowess in poses