A section from an interview with Geeta Iyengar and Lois Steinberg which provides insight on this practice:

LS – 'Pranayama is not easily taken up by students to practise. Could you describe the importance of a pranayama practice and how to develop it?'

Geeta Iyengar – 'I understand the problem. I think students feel pranayama is of one type just depending on the breathing process and therefore monotonous. To get interested in the subject is difficult and one really has to go deeper inside whereas with asana it is not necessary for a beginner to go straight away into oneself. That means a beginning is made from outside in. Therefore, a beginner as an extrovert can easily start doing some kind of practice. Whereas for pranayama , one really requires the inward-going process. Not only does one have to become an extrovert, but one needs intra-vision. Patanjali very clearly indicated that pranayama has to come after asana is conquered. In the asana there is a process of going from outside in. Making the body to understand, getting the sensitivity, the feeling of equilibrium, inner alignments, etc. If that understanding comes, then the pranayama begins.

The problem is that when students begin to do pranayama, they can’t see an immediate effect of it. So they feel that there is not feedback from the early stages of practice. Whereas from asana there is an immediate feedback: you feel nice; you feel something good happening in you; some life coming in you, blood getting circulated, feeling freshness of the mind; you feel that dullness is lessening. But in pranayama these things cannot be felt immediately. It takes its own time.'

Full article: https://iymv.org/geeta-iyengar-pranayama/