Coming to the specific exercise that the author has queried let me make it clear that this particular exercise has to be done slowly and only once a good practice is done then is to be continued. This exercise is technically known as Kapal Bharti and Swami Ramdev is credited for making it popular among the masses.
In this exercise one has to contract the stomach inside simultaneously breathing out. It will require some practice to learn it and one has to careful not to do it with rigourous intensity in the beginning itself. One has to contract the stomach a little inside and proportionately breath out. Do not try to do it with force as an experienced person does after 6 months practice.
If you start it softly like that then after a few months you would be able to do it in a powerful way and then would reap the real benefits.
I again warn that one should not do it with force in the beginning itself as that could be injurious.
Knowledge is power.
When we start anything we should do it in the same it is to be done in a systematic way. We have to do for a minimum time and slowly we have to increase the time. Then we will see get the benefits correctly.
A type of poison is being used in ayurvedic preparation and that will never harm our system. So the amount and the way you purify will make that as an ingredient in medicine,
I quote the relevant portion from the above answer.
"Along with this one can practice a simple 'yoga like' technique to draw and straighten the skin muscles.
Standing in a comfortable posture upright, take full breath gradually and at the point of starting exhale, pause a second, straighten up chest, draw the stomach skin inward while slowly exhaling and drawing more inward with the exhaling. The do the inhaling and aging the regulated exhaling drawing the stomach inward.".
More specific details can be had from the referred answer post.