There are a two exercises called Agni Sara that I am aware of. The first one involves "rolling" your stomach vertically by repeatedly sucking your diaphragm upwards and then releasing it while holding your breath out.
The second version of this exercise involves pulling your lower belly in a section at a time and releasing it a section at a time. I first learned this second variation from David Coulter's "Anatomy of Hatha Yoga." This second version may be helpful in developing better control of uddiyana bandha.
The first version exercises the intercostals since you use them to lift and expand your ribcage. It also stretches your diaphragm.
The second version trains your transverse abdominus and helps you to fine tune your control of this innermost abdominal muscle.
For both exercise, I would suggest doing them while on an empty stomach or at least a few hours after eating. Also, if you have blood pressure problems or are prone to feinting or have a hiatus hernia then you probably should consult a doctor before trying these exercises.
For all exercises you can do them standing or sitting.
For the first variation of Agni Sara, practice doing full exhales.
Take a relaxed inhale and then smoothly pull your belly back while you exhale. Smoothly release as you inhale.
I'd suggest exhaling through your lips but you can also exhale through your nose (provided you haven't got a cold.)
Once you are comfortable pulling your belly in as you exhale, focus on squeezing your ribcage inwards at the same time as you pull your belly in. Smoothly squeeze your belly and ribcage together to expell all of the air from your lungs and then smoothly relax and inhale.
Rest and rest when you need to.
Next practice using your ribs to inhale. You can sit down for this exercise and as you inhale focus on pulling your ribs up, away from your pelvis, smoothly and slowly. Expand your ribcage at the same time so that your ribs simultaneously lift and expand. (These can be felt as two slightly different actions.)
Focus on feeling the actual movement of your ribs since you will be using this same movement in the next part of this execise.
The next part of the exercise is also known as uddiyana bandha.
Stand up with your knees bent and with your hands on your thighs and your elbows straight. (If doing this while sitting, then place your hands on the floor.) You'll be using your arms as foundations to help you lift your ribcage.
Take a full exhale. Then lift and expand your ribcage without inhaling. Relax when you need to. Try to smoothly relax. Rest and then do it again.
Normally when you lift and expand your ribcage, you expand the volume of your lungs. This causes a vacuum which, if your throat is open, draws air into your lungs. Since in this exercise your are stopping the flow of air into your lungs, the same vacuum sucks your belly in and up. Actually, since your liver and stomach are suspended from the bottom of your diaphragm, your diaphragm draws these organs upwards. Your colon and intestines follow, causing your waist to hollow out.
After your exhale, you can push down with your arms to help pull your ribcage up. Focus on pulling your ribs up and on expanding your ribcage, without letting any air into your lungs. To release, relax your ribcage and then take an inhale.
As you get more comfortable doing this exercise, you can focus on expanding the front, sides and back of your ribcage evenly. You can also focus on drawing all parts of your diaphragm upwards so that your diaphragm is stretched completely.
Creating a vacuum with your ribcage and drawing your abdomen inwards is what Coulter calls Uddiyana Bhanda.
Keep all of the air out of your lungs and relax your ribcage. Your belly will relax and "fall" forwards. Then pull your ribcage back up. Repeatedly lift and expand your ribcage and then release so that your belly moves like a wave.
Relax and rest when you need to and then repeat.
Nauli Kriya is the name of the next variation of this exercise. It is where you work at making your rectus abdominus dance.
From Uddiyana Bhanda, keep your ribcage lifted but try to pull the front of your ribcage and pubic bone towards each other. This should make your rectus abdominus engage and pull forwards. Once you have this action, you can tilt your pelvis from side to side to give the impression that your are "churning" your belly.
Take regular rests when you need to and repeat.
If you can bend your lumbar spine back and forwards (and side to side) while doing this, it may actually massage your lower intestines and that may help you with your morning poo.
To test whether this is true for you or not, practice regularly for a week or two weeks and notice the regularity of your bowel movements and how you feel in general. (You might want to keep a journal or "log".) Then try a week without and again record your findings. You may have to control factors like what you eat and drink. Notice what other factors may or may not affect your bowel movements.
Failing any affect on bowel movements, this exercise is an excellent party trick and more seriously, it is a good way of practicing control of your body.
And now for the second method of agni sara. (Note that Coulter suggests doing this exercise while standing. I first learned this exercise while sitting and that is the way I continue to do this exercise.)
While sitting slightly slumped, slowly expand your belly. Slowly pull it in and back. Try to make both actions smooth as well as slow. Keep your ribcage still as you do this.
I suggest sitting in a slightly slumped position so that your abs can relax.
For this reason, you may find that sitting on the toilet is a good time to practice, especially if you find yourself "waiting for delivery."
Now, once you are comfortable with pulling your entire belly in and then releasing it, work on pulling your belly inwards a section at a time. Divide the part of your belly below your belly button into three horizontal bands or strips. The portion of your belly above your belly button you can divide into two horizontal bands.
Focus on the lowest band. Slowly pull it in, and then slowly release it. (I found the releasing part to be the hardest.) Do this without contracting the other four "bands". Next pull your lowest band in and then pull the second band in. You can then release the second band and first band sequentially, or you can practice on activating and releasing the second band while keeping the first one pulled in.
Add the remaining three bands in the same way.
You may find yourself focusing on a part of your belly that you can't quite control.
Rest if you find yourself getting frustrated or running out of time.
Next practice pulling each layer inwards sequentially from bottom to top. You can slowly release all at once.
You might pull band 1 in, hold take an inhale and then on your next exhale pull band 2 in. And so on till you reach the 5th band. Then work at releasing them from top to bottom.
For your reference I should again mention that I found releasing these bands of muscle sequentially more difficult than engaging them. These bands of muscle are all a part of the Transverse Abdominus. You are thus practice control of this muscle.
You may find that after practicing this version of agni sara, your lower belly feels slightly energized and that you have a pleasant "buzz." Another benefit is in the context of whole body breathing where you are using your abdominals, diaphragm and intercostals all together to drive your breath.
And it may help you to better activate uddiyana bandha.
If you can pull your lower belly in while inhaling you may find it easier to breathe into your upper belly and ribcage. It is as if by pulling in your lower belly your give your upper belly and ribcage room to move… room to breathe. You can play with how much of your lower belly you pull in and how it affects your breath. Try with just the lowest band pulled in and then the two lowest bands. Pull in smoothly and slowly as you inhale into your upper belly and ribcage. Relax while exhaling. Notice how your breath feels. Adjust accordingly.
You might want to try practicing both variations of agni sara regularly as part of your "get out of bed" routine. It can help your bring your awareness to your core and then you can radiate your awareness outwards from there.
As a yoga teacher, I'm constantly exploring new exercises, new ways of doing yoga poses.
There is no single "right way" of doing a yoga pose. Instead, there are options. And the better you are at "feeling" your body, the better you can get at choosing the right option for your body as it is now.
For any technique, the point of practice is to learn feel it and to control it, so that it can be used without thinking about how to use it.
And that is more or less the approach taken in all of my ebooks and videos. They help you to feel your body and control it so that you can work towards using it effectively in anything that you do.