Bahya is Sanskrit for “outside” or “external”. The name of course comes from the breathing technique itself. You are breathing out and then holding your breath. And that is the basis of Bahya pranayama. A strong breath in and then a stronger breath out is the essence of this exercise. Focusing on this breath can help with anxiety and strengthens your diaphram. Pranayama breathwork is also great for mindfulness.

This is the breathing exercise where your stomach looks as if it has been sucked in. Or your stomach has been hollowed out some. That is from the contraction of your abdomen because this exercise calls upon the practitioner to fully empty their lungs by pushing air out with their diaphragm.
So how do you do this.

Sit comfortably in a cross-legged position sometimes called Lotus pose.
Breath in deeply and then exhale deeply
Try to pull your abdomen up and in as much as possible especially in the area just below your belly button?
Move your head down so your chin touches your chest and hold the breath here for 5–10 seconds.
You can focus on exhaling negativity on your exhale.
Repeat 5–10 times for full results.